I spent about 18 months periodically answering questions on Twitter from people asking if people you'd blocked can still see your tweets. The answer is always 'yes' and that hasn't changed. These questions were posed around the time when Twitter made quite a few changes to the way the block appeared to work, but the actual effect was very dependent on the app that you use to view Twitter.
If you try and view the tweets of someone that's blocked you from an official Twitter app (eg Twitter for iPhone) you won't manage it and it looks like the block is much stronger. But if you view on a third party app (Echofon for iPhone, Fenix for Android, Dabr for desktop) then you can see and reply to their tweets. And they can do the same to yours. So the block is app-dependent and doesn't stop anyone from being able to see anyone else's tweets. Plus everyone can log out and view them anyway.
So why block?
Keeping your follower list tidy / minimising pointless Notifications
This is the number one reason I block people (often reporting as spam before blocking them). Since I began using Twitter in 2008 I have regularly pruned the list of people that are following me or that interact with my tweets.
Everyone experiences phases when bots or fake accounts start interacting with your account. Sometimes they'll follow, but more commonly they'll favourite a tweet. This gets your attention in a fairly low-key way but it's annoying (notifications!) and I think it's important to report as spam and block so that Twitter can remove them. I know this can work because often (not always) when I check back later the account's been suspended.
Sometimes these accounts look extremely convincing at first glance but if you begin to see a lot of them you soon recognise their characteristics.
Twitter is overrun with these spam accounts that retweet stuff randomly. All use unconventional separators ✦ etc pic.twitter.com/QZx2Ila137— Jo Brodie (@JoBrodie) January 29, 2016
Intrigued by these accounts, reporting and blocking several of them every week. pic.twitter.com/e2elNecUwl— Jo Brodie (@JoBrodie) June 25, 2016
Incidentally I reported both those particular accounts for spam and blocked them but they're still there so Twitter disagrees with me (they are spam accounts but easily pass under the radar. One's not tweeted since April, the other not since July).
Once I posted something fairly innocuous about Afrezza (an inhaled insulin for people with diabetes) and began to notice unusual behaviour on the tweet and replies. They were being favourited and retweeted far more frequently than was warranted so I ended up blocking everyone involved just for some peace and quiet. It seemed to be some weird targetted thing where these accounts tried to boost anything Afrezza related.
@mpg5454 @Vanderubble I mostly see odd spam-like behaviour on Afrezza-mentioning tweets, from egg-avi accounts pic.twitter.com/fsT8mchV8T— Jo Brodie (@JoBrodie) July 7, 2016
Note 'egg-avi' means having an egg for the account's avatar / picture - while it's not a guarantee that an account is spam it's certainly a marker for it.
I've blocked (often pre-emptively) all of the Right Relevance accounts (there are hundreds of them). They favourite or RT your tweets if you mention a particular word that the bot is monitoring so you can end up with lots of tedious notifications (only on Twitter, I switched off the email thing years ago!). I consider them to be spam but they do provide a service of sorts, boosting tweets about a particular topic, which you may find useful.
There seem to be quite a few of these spam accounts pic.twitter.com/TzPrqc54KI— Jo Brodie (@JoBrodie) February 19, 2015
Some accounts retweet genuine tweets, though never post anything of their own.
Here are types of accounts that I block and / or report for spam pretty much automatically
- Egg avatar plus a name with a random string of alphanumerics
- Accounts that only retweet tweets, never post their own content
- Accounts that follow 100 celebrities and me, or follow hundreds of people all called Jo
- Businesses who sell 'widgets' who follow me after I've posted an unrelated tweet mentioning widgets
- As a bonus, third party apps also tell you what platform someone is using to send tweets - this can indicate if a tweet's been sent using some automatic system
Although blocking someone doesn't stop them from viewing your tweets it does make it much harder for them to see who you're following and who's following you, so blocking a spammer in this way stops them following others in your lists.
Blocking someone means you don't have to see their tweets if you don't want to
They won't be delivered to your timeline or mentions (in some cases you might see them if someone you follow retweets them). Muting actually does the same thing (if you don't follow them) and it has the added bonus of them not realising as they can still see your tweets. I think people use muting as a sort of fun passive-aggressive block.
Blocking someone stops them seeing your tweets
No, it doesn't - they can log out, use a spare account or a third party app. This is a bad reason.