No you don't have to get them. If you do though you get some unlocks. However, you can just put the code in at the computer screen "3ARC INTEL" and it will automatically give you the intel packages and unlocks.
Collecting all 42, Intels you will unlock the Closer Analysis Achievement/Trophy ..... Hardline Pro, Change the contents of a Care Package ...
Enter "3ARC INTEL" then "YES"as a code to unlock all Intel NOTE: enabled you cannot earn the achievement for finding all intel in Campaign
Collecting the intels is just like a side mission. It won't affect the progression in the game. It's hard though, I've only collected 6. As jeremy said, collecting all 42 intels, will give you the CAA trophy ( Closer Analysis Achievement ).