Mobile check-in has a couple of perks. I've used it more on the Legacy Marriott side.

1) If using the same credit card as your reservation, you merely need to show ID to get your keys, no signing or swiping or PIN entry.

2) If the hotel is nice, or if you specifically use the chat function (exists in Marriott app, not about SPG), sometimes you can fish for a bigger upgrade. 95%+ of my stays I don't bother asking if an upgrade is available, but when an upgrade would be really nice (e.g. 3+ family members in the room with me) or a particular amenity (example: room with an actual freezer because I had a contusion on my tibia and gel ice packs that I could use if I had a real freezer) they'll usually oblige if at all able.

3) As a subpoint to #2 , if you do mobile check-in at least a day in advance and ask for an upgrade - if any upgraded rooms are available, you are more likely to get them. Since on the Marriott side (until the merger finishes) there are no SNAs, use of the mobile check-in on a Sunday for a stay starting on Monday effectively allows you to "get in line" sooner for the upgrade. If your location attracts a lot of business travelers with elite status, this can make it more likely that you get an upgrade before other travelers check in in-person Sunday afternoon/night.
(This is highly variable by hotel; some hotels will only process mobile check-ins on the day of, close to the listed check-in time. Others, I've had a "your room is ready, you've been upgraded" prompt the day before.)

4) Particularly if I want a very early check-in (think 10AM-12PM usually the hotel is more likely to oblige if I make the request at least a day in advance).

5) Mobile keys at participating properties theoretically make the stop to the front desk totally unnecessary if you're taking the points instead of the $10 F&B voucher on the Marriott side as an elite, although I have never gotten this function to work properly and wouldn't be comfortable only having my hotel key on my phone anyways (what if it gets lost/screen broken/dead battery?)

I usually do mobile check-in for reasons #1 and #4 and don't make any special requests (ask for an upgrade or late checkout, etc.)