OT call uber just before you need them?

IIUC, with Uber, or Lyft, one is supposed to use the app just before one needs the ride, and NOT make an appointment 12 or 24 hours in advance.

But how do I know if anyone will be working near me at the time, Sunday morning at 9:30 when I have never used the service before.

And when last night I thought I could check it out, but it looked like I was about to agree to a ride right then, even though I only wanted to see how many cars were nearby? A screen came up where I was to promise to pay. Nothing else is like this. On Home Depot.com you get to look at what's for sale before you agree to pay for it.

But even if I've misunderstood this, the bigger question remains, Can I use uber or lyft 23.5 hours in advance and say I want a car here tomorrow morning. Like i could with a taxi.

P.S. I signed up for uber last night and between 7 and 8 this morning I got two spam emails with lines like these in the headers:

From: "MyBackPainCoach " Subject: The REAL Reason Behind Your Pain... Reply-To: "Uber"

Subject: Relieve your back pain in just ONE session! Reply-To: "Uber"

But it comes to an address I iddn't use for uber. I noticed yesterday that one webiste knew several of my addresses, maybe it was uber. Anyhow, do you know if this is just a coincidence, is uber sending me spam, or what? (I didn't want to enrol in uber but that seemed to be the only way to check it out.)

I misunderstood this because today it does show a map with cars before I agree to pay. Last night I guess there were no cars in the area, so the map showed none, and that is exactly the problem. Today there was just one car on the map, which showed half of Baltimore, all the way down to downtown. Maybe they only show those that are close?? but if there is only one now, there might be none on Sunday.

With a taxi company they find a cab who will take the call and if it means his starting to work earlier than he would have, he does. Or skipping a fare that would conflict, he either skips it or checks that the dispatcher can find someone else pick me up.

But with uber, it seems that one is stuck, and now it's so late it will be harder to get a cab. ??