You arent the only ones who had been experiencing this problem.Take a look with this user which does have similar problem of yours.
https://www.reddit.com/r/Telegram/comments/8dqkma/how_do_i_block_continuous_fake_users_joining_my/How is it for anyone in any way economical to do that? what is the purpose of that?
On situations like these where these bots do join up tons of existing groups or channels is for the purpose of spamming out their scam links or referral links in lots of ways.
Other purpose is that other groups does even like to have some show-off of lots of users in the channel for it to believe that they do have lots of supporters which
can be possibly be used as an alibi and do up some shady things and stuffs.