Trip from Paris to Mont St. Michel

The main benefit of spending the night is to be there after the day-trip crowds are gone and get into the Abbey when it opens. If you're an early riser, you might even make it up the hill for the early-morning Lauds service (must arrive 10 minutes ahead, when the door is open).

Train/bus connections take 3 hours each way when you search for destination "Mont St Michel" and the timing works pretty well for round-tripping in your direction (from Paris). You'll get a few additional options if you search to "Pontorson-Mont-St-Michel." Of course, that means visiting MSM during the most crowded time of day.

Looking Up Train Schedules and Routes Online gives you the Deutsche Bahn train schedule link and tips for using it.

One doesn't "need" to spend the night, but this attraction is so crammed with tourists during the day (shoulder to shoulder -- it's ridiculous), that if you have the option to spend the night, you will enjoy a completely different and sublime experience, walking quiet, deserted stony streets and viewing the distant lights of the shore.

I've only been once to Mont Saint Michel. My wife and I spent one night, and I can barely recall my time there before sunset. Perhaps I'm blocking it out due to its unpleasantness. But I believe I will go to my grave recalling the peaceful, calm, and still twilight and evening I was able to experience by staying overnight.

In my view, it's the only way to properly experience the island.

Spending the night is one of Rick's main crowd-beating strategies in general. For extra schadenfreude, go watch the crowd-weary tourists trudge to their buses at about 5 PM.

Coming from Paris to MSM only over one day lets you few time on the spot, and with quite many other people from 10am till 5pm, at least in summer (from October to April/May, it is much more quiet). For those who can afford more time, spending the night right on the rock is a pleasant solution but not the unique possibility. If you have your own car, you may also try to arrive early in MSM from Paris or (probably better) from a town/city in Normandy or Brittany (Rennes, St Malo, Bayeux, Caen...).