In most gyms, a spotter is not necessary for safety. In all lifts except bench press you can just dump the bar, and you can bench in the squat rack and set the safeties up so the bar won't squash you.
Some people like spotters to do forced reps. There are a couple of guys at my gym who show up every week or two and help each-other with forced reps on every single rep of every single set. Rather than shoulder pressing 20kg dumbells with 5kg of force from your buddy, it's probably easier to shoulder press 15kg, I would have thought. But whatever makes them happy.
Normally I would only expect to see 1-2 forced reps at the end of the last set, and not otherwise, and a spotter is useful for that. But forced reps are not really possible for deadlifts, bent over rows, and so on - and yet people manage to get stronger on those exercises. So while it can help, you can probably do without it.
Otherwise, a training partner is very useful. You encourage each-other, you have greater confidence in trying to personal bests, and so on. But as others have mentioned, it's hard to find a good training partner. They need to have similar strength, goals, and training style to yours.
They don't have to be identical. If I'm benching sets of 40kg warmup to 80kg top weight and my buddy is doing 60kg to 100kg, it's not hard to add and remove plates. If he wants more size and I want more strength, well he can do 10 reps while I do 5 reps, so what, and he can go home and eat a couple of whole roast chickens.
But if he's doing supersets of biceps curls and dumbell flies and tricep pushdowns while I'm doing squats, bench and deadlift, well then we don't really have a common ground.
Finding a decent training partner is actually harder than finding a decent professional trainer, I've found.