There are two very different ways people try to end up in a better train seat, and it's worth being clear on which is which before you're standing in a train corridor at 11pm trying to figure it out.
Option A: Ask the TTE to move you
This is the option most travelers default to because it's the one they know exists. You approach the Travelling Ticket Examiner and ask if you can move to a different, unoccupied seat. It can work — but it depends entirely on the TTE's discretion and what's actually free in the coach that day, it's not a guaranteed service with no formal process to request it in advance, and it doesn't help if the seat you want is occupied by someone else who hasn't offered to trade. (For the underlying rules, see what IRCTC and the TTE can and can't actually do.)
Option B: Swap directly with another passenger
This is a mutual, consensual trade — you take their seat, they take yours (or a chain of swaps involving more passengers). It's a long-standing, informal but accepted practice on Indian trains, typically done with the TTE's awareness so the manifest stays accurate. The advantage over Option A is that it works even when every seat in the coach is occupied, because you're not waiting for a vacancy — you're trading with someone who has what you want.
The traditional problem with Option B has simply been discovery: you have no way to know who else on the train might want to swap, short of walking the coach and asking. That's the gap CoSeat is built to close — verify your PNR, state your goal (say, landing a lower berth after missing the quota), and get matched with a co-passenger on the same train (or flight) who's open to trading, with the swap only going through once you both agree.
Bottom line: asking the TTE is worth trying, but a direct swap gives you far more control over the outcome — you're not limited to whatever happens to be empty.