Logged-in profiles are used to identify certain players anytime and anywhere. If you use such profile, server can be sure that it's really you (unless someone has stolen your password or secu_login key). This is very important especially for assigning additional privileges to players (admins, moderators, etc).
CryMP supports logged-in profiles from the beginning. In the old days players without their own profile got just randomly generated profile ID from the range 800000 - 999999. These players had different profile ID every time they launched the game. Master server also didn't validate these random profiles, so they were quite useless. Lately today's so called static profiles with 100xxxx IDs were added. These profiles are automatically assigned by the master server and are somewhat bound to the computer of each player. That means it's now possible to identify all players and not only those having their own registered and logged-in profile. It also fixes one nasty problem. As you probably know, you can register as many profiles as you want, so logged-in profiles are not so useful for banning someone.
The problem with static profiles is that they are not as static as they should be. Your static profile ID can actually change sometimes (rarely) because there's no 100% way to clearly identify players without their own effort. However, static profiles are still quite safe and also spoofing someone's static profile is very hard mainly because servers validate the info sent by your multiplayer client on the master server. Although, as far as i know, only servers using SafeWriting or CryFire actually do such validation, but that's another story.
Conclusion: Logged-in profiles are for giving you some special privileges on the server. Static profiles are for banning you from the server. Today, correctly configured servers can obtain your static profile (together with some additional information) even when you use logged-in profile, so there's no reason to not use your own logged-in profile. It's actually a nice thing because it shows that you're not too lazy to register and configure your own profile, and also server operators can easily give you, for example, moderator privileges because from a technical point of view you already have everything required for that.