Mulching leaves into the lawn is tremendously beneficial for several reasons:
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It provides organic matter to the soil (good for nutrient and moisture retention, alleviates compaction, and improves drainage in the long term)
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It provides the lawn with many nutrients that are difficult and expensive to supply otherwise... Particularly, but not limited to, all of the micronutrients. (Trees are just way better at taking up nutrients than grasses are)
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Mulching is an incredibly effective form of pre-emergent weed control... Extremely effective for preventing broadleaf weeds, and can even prevent/reduce future poa annua and crabgrass.
According to MSU, up to 6 inches of leaves can be mulched into a lawn at one time. That number partially depends on your mower performance... But even in the worst case scenarios, it might just mean going over the leaves multiple times. (Still quicker than raking or bagging)
Tips for mulching leaves effectively:
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go into fall at a high mowing height... Its too late to change that now, but it helps.
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use an actual mulching blade (most new push mowers come equipped with mulching blades. Mulching blades are the ones with the curved cutting edge and the blade has curved surfaces on top to generate uplift)
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plug the side discharge chute. Push mowers usually have a flap that's easily closed. Riding mowers often require a seperate accessory to plug the chute.
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don't let the leaves pile up. Most of the time, weekly will be enough, but if you have windy days, you might need to get out there an extra time.
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do it when the leaves are mostly dry. It can actually help if they're a LITTLE wet... But dry is certainly better than too wet.
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if you notice clumps of matted leaves... Knock them loose. I usually just kick them, but a rake or blower works too.
The classic argument against mulching is "they'll smother the grass"... Simply put, if you smother the grass, you're doing it wrong (especially that last step)... Unless you've got a lot of poa trivialis or poa annua... Mulching leaves can actually smother those... In which case, that's usually a good thing... But even then, they'll still fill back in next year.
A quick note: Don't mulch leaves if you plan on dormant seeding... The weed prevention thing I mentioned above also applies to ungerminated grass seed.