Wanting to practice and having a piano in front of you are two different problems. Maybe you don't own one yet. Maybe you do, but it's in a room where you can't make noise at 11pm, or you're on a train, or the family is watching TV right next to it. The practice you actually get done is the practice that fits your real life, and a phone fits almost anywhere.
So here's what you can genuinely work on without a real piano, and the bits where a screen falls short.
What a phone is good for
Three things, and they happen to be the three that matter most when you're starting.
The first is knowing where the notes live. Playing on a screen keyboard builds the same mental map as a real one: which way is up, where the black keys cluster, how far your hand has to jump. It isn't the same as weighted keys under your fingers, but the geography transfers.
The second is rhythm and timing, and this is where a phone genuinely shines. A falling-note mode like Piano Game trains you to hit notes in time. Slow a song down while you learn the movement, then speed it back up once your hands know the way.

The third is your ear. Hunting for a melody you're humming, or hearing whether a chord sounds happy or sad, needs no special hardware. Ear is the thing most self-taught players wish they'd built sooner, and a phone is perfectly good for it. There's more on this approach in how to play piano without reading sheet music.
