Well, some vets will go a step further and want to test a stool sample yearly FIRST...well on one hand then if the sample comes up clean then you avoid giving unnecessary meds...but on the other hand - given the choice between preventative deworming or testing samples just "because", I would choose to deworm w/o the test. I hope that makes sense. I think yearly stool checks for indoor cats is a bit of overkill....others feel differently. Stool checks are not 100%, either...
If I wanted/needed worm meds from my vet, they would just sell them to me w/o an exam, if they had seen the cat oh, anytime in the past year. The meds run me about $10 per cat (2 doses) so I wouldn't say that's much of a profit maker for them.
So, no I don't really think the vet is just trying to get money.
No, I don't necessarily think it's necessary to deworm on a regular schedule (for indoor cats).
But, yes...depending on the environment, an indoor cat can get worms much easier than one might think...by eating fleas (even one, if it's the right one), mice, bugs, etc. We can even bring the eggs in on our shoes, from the grass.
So if I see the slightest clue that one of my cats has worms, I do deworm.
I don't think the medicine is harmful if it might be needed, but if it's not needed I would not give it. After all, it is worm POISON...so I try to be somewhat cautious with the frequency, based on necessity.
I hope that helps.