A regular clean keeps a home in good shape: counters, sinks, toilets, mirrors, floors, dusting of the surfaces you can see, and trash out. It's built for maintenance, and it works best when the home is already at a good baseline.
A deep clean is what creates that baseline. It reaches baseboards, door frames, light switches, cabinet fronts, grout lines, vents, ceiling fans, and the top of the range hood: the places where grime builds up slowly enough that nobody notices until it's obvious.
If your home hasn't been professionally cleaned in a few months, if you're moving in or out, or if you're starting recurring service, begin with a deep clean. Every regular visit after that is faster, cheaper, and more effective, because it's maintaining a clean home instead of chasing a dirty one.
A good rule of thumb: book a deep clean once or twice a year, and regular cleaning weekly or every two weeks in between.