it is a prerequisite for the deck?

Mulligan and combo are settings written to utilize the full potential of the deck through external A.I such as silverfish. If the deck you're playing with has a lot of synergy or can easily be misplayed then it is best for you to get those files in order for the bot to not make stupid mistakes (Like for my mechmage deck the bot would throw down Goblin Blastmage before any other mech thus missing the 4 extra damage). Settings is the file that governs the depth of calculation for the A.I (such as how many boards it will read ahead or how hard it will try to play around secrets, etc) and you also have other setting as well. I see that this is your first post and you might not have an external A.I silverfish, before setting everything up I suggest you search silverfish on the forums and there should be a nice guide on what it is and how to use it!