Cat toys are very important for cats. They way they are designed is so that they can replicate movement and sight like cats would see in the wild. This creates an environment that they can play in and hone their instinctive skills such as stalking, hunting and catching their pray. They also need regular excercise this is most important if they are indoor cats, as they can get bored and may be prone to getting fat and unfit.
However you don't actually need to buy cat toys you can make your own simple ones, like bits of string, or add something to a pole and make into a fishing rod style toy, you can use a scrunched up bit of paper to tie on, anything most cats will like to play with. Basically anything that moves fast so that they can watch and use their instinctive skills to the full.
My cats have bought toys and made up toys and they play with both.
Their favourtes are ping pong balls, small furry mice and a home made fishing rod style toy with a bit of string on that they like to chase.