Beacuse you need something to support the rim. If you didn't have spokes, you'd have to have a solid disc. (well, maybe with some cutouts, like an alloy car wheel) and that'd be a lot heavier and more sensitive to side winds.
Spokes are found as part of wheels, for instance on a bicycle. They connect the hub to the rim.
Without spokes, wheels would have to be made from solid discs.
The wheels
"Spokes" are the things that branch out from the centre of wheels to the outside edge of the wheel, to stabilise them. So wagon spokes are the things that branch out of wagon wheel centres (usually wooden, in this case). Bicycle wheels also have spokes.
360 / 15 = 24 spokes
A hub is the center piece on your wheels that the spokes are connected to. it spins your wheels. if you dont know what spokes are they are the stick looking parts on your wheel.
"Spokes" are the things that branch out from the centre of wheels to the outside edge of the wheel, to stabilise them. So wagon spokes are the things that branch out of wagon wheel centres (usually wooden, in this case). Bicycle wheels also have spokes.
Bicycles need a reflector on the spokes of the wheels.
That would differ from one bike to another. A really small bike with maybe 12" wheels will have fewer spokes than a bike with 20" wheels. Say between 24 and 36.
This completely depends on the wheel. The most common number of spokes per wheel is 36. Some better wheels have 32. Higher performance wheels have even less.
Because before the days of big milling machines and huge lumps of aluminum the most available way of making wheels that were true, strong and reasonably light was by using spokes to lace a rim to a hub. For bicycles, with their relatively large diameter and lightly loaded wheels, the method is still competitive.
Bike wheels are made out of three parts: 1 The center hub, the nave , 2 the rim, 3 the spokes. The spokes have hook's on one end, that are latched through the holes of the nave . The other end of the spokes are threaded and are put through holes in the rim and then fastened with a nut with a flattened head. This was the easy part. The difficult part is to tighten all the spokes equally so that the nave and the rim are in perfect alignment.