Putin's approval rating since the war began has been a steady 80%+ (https://www.statista.com/statistics/896181/putin-approval-ra...). We don't know yet what September's numbers will look like, but it will likely take a hit from the retreats and losses of territory to the Ukrainians this month. Mobilization is going to drop that further. People are ok supporting a war in the abstract, but less so if they have to go out there and fight themselves. And they'll like it even less when they find that people with connections aren't being called up.

Calling for mobilization now gives them a few months to train and get equipped when winter brings fighting to a close. It'd be interesting to see where Russia finds officers for these new troops. Or maybe training/equipment/officers don't matter, if the plan is to use them as cannon fodder.

Regardless, Ukraine will find it harder to make progress next year like they've done in the last few weeks. There won't be any fronts that are thinly manned and easy to punch through like Kharkiv was.