Basically, the season passes unlock the operators without needing to grind the points to do it. Each year has a new group of operators and they cost 25k each to unlock. I got the base game, so I only needed 500-2000 to unlock the ones it came with, but starting with the DLC operators they were 25k each.
So, if you don’t care about the new operators, you don’t need it. If there’s stuff besides the operators, it’s also not necessary. You also use points to unlock charms and skins, but you don’t need those. The only other thing is attachments for weapons (suppressors, sights, grips) which affect weapon usage, but they’re cheap (100-250 each).
I never bothered to get any season passes. I have about 80 hr in and I’ve managed to unlock all the base game operators and 3 of the extra ones, all playing coop T Hunt only. There are 11 right now I think.
However, if you want the chance to try them all without grinding, then by all means spring for a pass. Assuming you like the base game, of course!