Blogger users do not really need it, true. However, for those who host their own site, the amount of spam comments is very high.
I have to moderate 5-10 comments per day on my hosted blog, and none of them are legit, What is worse is that The spammers post them on older articles, hoping you will miss it.
The purpose is to put content out there on your blog so that they will get referrals, and look more relevant in searches.
It is annoying to have to type "BEIb3r GraNf67" just to say "great post" but especially within other team members blogs, I want to continue everyone to comment anyway. It is not a rule, just nice to have.
If you have a blogger account which is monitored for spam already, Consider removing the Capcha and other security, and see if your comments go up!
-Mitch