One reason is that the military requires it. So if you want to compete in military trials you have to have it.
A lot of 1911 fans like the thumb safety; and want their other pistols to be like the 1911. Personally, I like an exposed hammer like the HK has. I do not like having to place my thumb on top of a thumb safety ( a requirement in all tactical courses). It is uncomfortable for me. So I like the striker fired pistols without the thumb safety.