Tyre tread won’t improve grip on hard surfaces, even when wet. On soft surfaces (like grass, mud or snow) the treads of city bike tyres are pretty much ineffective. You’d really need knobs with proper spacing.

So why do city bike tyres often have treads like these:

(photos show Schwalbe Marathon and Continental GP4Seasons tyres)

I can see several possible reasons:

1. Marketing and customer expectations. Tread just looks … grippier. I’m pretty sure that’s the reason why even road bike tyres often have tiny, useless treads on the rubber surface. Doesn’t increase costs and doesn’t have any other disadvantages, so they just do it.
2. For thick rubber it allows the rubber to flex without cracking and probably reduces rolling resistance.
3. Maybe some weight savings without sacrificing too much puncture protection.

Tyre tread won’t improve grip on hard surfaces, even when wet. On soft surfaces (like grass, mud or snow) the treads of city bike tyres are pretty much ineffective. You’d really need knobs with proper spacing.

So why do city bike tyres often have treads like these:

(photos show Schwalbe Marathon and Continental GP4Seasons tyres)

I can see several possible reasons:

1. Marketing and customer expectations. Tread just looks … grippier. I’m pretty sure that’s the reason why even road bike tyres often have tiny, useless treads on the rubber surface. Doesn’t increase costs and doesn’t have any other disadvantages, so they just do it.
2. For thick rubber having those “cuts” (negative tread) probably allows the rubber to flex without cracking and maybe reduces rolling resistance.
3. Maybe some weight savings without sacrificing too much puncture protection.