Why are we given the ability to change our title (Street Thug, Master Street Thug, etc. under the Profile tab)?

You earn new titles as you master job tiers. You can tell (roughly) from someone's title what level they might be. Someone who has a Boss title, for example, must be well over level 100. You can also get a rough estimate of how strong they might be! Someone who is a Master Boss, for example, has probably has picked up a few Private Jets, Federal Agents and other good equipment along the way, so you may want to be wary.

When you master a job tier, your title will be automatically set to the tier you just mastered. You can use it to show off your level in an effort to scare off would-be assailants, by making it clear what your highest job tier attained is.

However, changing your title to something much lower than what you truly have attained can benefit you. By disguising your true level, you can lull weaker players into attacking you so you can win (especially when you are on the hitlist!)

I personally always leave my title as something low level, like Associate Street Thug. When I am hitlisted, usually a few low level players will attack me and lose, meaning I gain experience. Sometimes, it's enough experience to offset the 6 experience I lose when snuffed! This is increasingly the case as I level higher -- I now very rarely get killed from the hitlist and lose experience overall.

Getting experience via the hitlist is called 'riding the hitlist' and is a very popular strategy to gain experience among higher level players. Changing your title to something low level is one tactic you can employ to attempt riding the hitlist at lower levels.

If you want to keep your title set to something low, do not forget to change it back after mastering a job tier -- the game automatically sets your new title by default when you achieve it, so you have to change it back if you want it set to something else!