There’s no “must” in how to summon a succubus/incubus, and there’s a lot of methods to summon these entities. You can even do a Petition Spell as described by @DarkestKnight.
Some people prefer to use a protection circle, probably as a use to “ease their mind” and build up courage and self esteem for themselves. If that will keep them “warded” and the spirits on a safe distance is up for debate. In my opinion, it might remove the purpose to instigate physical connection. Unless it’s an “Interview With A Succubus” you’re after, or to ask her to do your bidding? 
Keep in mind that I’m not saying it’s a lesser method to summon a succubus, but I haven’t done it either.
Yeah, that was the method I used back in the days.