Currently there are dozens of alternative coins. Some are just "copy&paste" coins, some do have a slight modification of the algorithm or some small extra features. However, in my opinion neither of these coins has a real advantage over bitcoin. Thus, why are you mining/investing your time into one of these altcoins?
Because GPU miners in particular can make a lot more money mining scrypt based altcoins than mining bitcoin. Litecoin is
regularly in the realm of 100-150% profitability according to various altcoin comparison chart / stats sites. There are several other coins lately that have exceeded 200% profitability and stayed up there for a few days at time, far better than BTC even when they eventually pop in some cases.
So the short answer to your question is:
"to make more money".ASIC investment remains high risk, many won't ever see ROI. Many miners don't want to get rid of their GPUs just yet for various reasons.
If your idea of mining is squeezing every bit of coin out of your GPU hardware then altcoins are definitely for you. On the other hand if your idea of mining is to setup an ASIC or mine BTC shares with a GPU and check on it once a week or less then you have probably wasted your time reading this thread because altcoins are not for you.
Whether its worthwhile mining the other SHA-256 coins with an ASIC is another matter entirely that I don't think I'll go into right now...