Typically when you see mass automation of benign posts they are "priming" the accounts to make them appear human and trustworthy before they start spamming/phishing with them later. Brand new accounts are treated with more scrutiny by spam filters so this technique lets them get around that.

A more primitive version of the technique was YouTube comment bots that simply re-posted other users comments verbatim - now with LLMs they can easily generate unique white noise posts instead.

In India, there is a lot of competition in the tech field so devs often want to put open-source contributions on their resume. This definitely does not account for the majority of the spam, but could possibly be a sizeable minority of it.

Back in the before times it wasn't uncommon for people to release malware without any explicit monetary benefit. Sometimes they just did it as a "fuck you" to Microsoft, who at the time had a very very poor reputation for a myriad of reasons beyond the scope of this comment.

tl;dr they might just do it for the lulz

Scammers applying to 10 jobs have to have accounts that have activity.