Basic speed question 2.4 GHz vs. 5 GHz / also type of security

In setting up the divice, I noticed there were 2 speeds of 2.4 GHz & 5 GHz. Why would you not always want to connect to the higher 5 GHz. It may be due to the router capabilities but it came without an instruction book, and speed information was not on the router. On some devices I see the option of connecting to either speed, but on this desktop I only was able to see the 2.4GHz.

Also there was a selection of protection of WPS and 2 others. Unfortunately I did not write the other 2 down, but chose the 2nd one and not the WPS. Too bad there was not some basic definition of what each of the 3 potention levels involved.

Any basic information would be appreciated.