In addition to the valuable points made by Rider_X, I suggest two other reasons for learning to stand.

Starting on hill can be difficult if you cannot stand up and pedal. So being able to stand gives you more options for where you can start from.

More importantly, it's a safe bike handling skill. Riders sometimes hit a bump that throws them off the bike, or at least jolts them enough for them to lose control. This occurs when the rear wheel gets to the bump, and the seat gives your bottom a kick. It's happened to all of us, but it needn't.

When coming to a bump, get your bottom off the seat. Easily done if you are confident to stand on your pedals. By standing, you allow the bike to pivot over the bump without giving you that dangerous jolt.

So, not a hard core skill, more of a safe riding skill.

More hard core riders also get their bottoms off the seat on fast descents, when they don't know or can't see the quality of the road surface ahead. Hitting an unseen bump at high speeds can easily cause a crash. And jumps are another case, but I digress ...

In addition to the valuable points made by Rider_X, I suggest two other reasons for learning to stand.

Starting on hill can be difficult if you cannot stand up and pedal. So being able to stand gives you more options for where you can start from.

More importantly, it's a safe bike handling skill. Riders sometimes hit a bump that throws them off the bike, or at least jolts them enough for them to lose control. This occurs when the rear wheel gets to the bump, and the seat gives your bottom a kick. It's happened to all of us, but it needn't.

When coming to a bump, get your bottom off the seat. Easily done if you are confident to stand on your pedals. By standing, you allow the bike to pivot over the bump without giving you that dangerous jolt.

So, not a hard core skill, more of a safe riding skill.

More hard core riders also get their bottoms off the seat on fast descents, when they don't know or can't see the quality of the road surface ahead. Hitting an unseen bump at high speeds can easily cause a crash. And jumps are another case, but I digress ...