Evening
Generally the concept of a truename is used to grant the user of that name great(er) power over the named thing.
I've seen this treated as running from being a requirement for summoning, binding and controlling demons in some games, through to working similarly to a power component for such spells in others.
IMC I tend to use truenames as a means for adding a background to dealings with the larger supernaturals; so using the truename becomes a very powerful bonus to your dealing with them (whether through bonuses to spell use, spell penetration or just to skill checks when dealing with them) - meaning powerful to the extent of making the difference between a roll (of whatever type) being passable by a 10th level character rather than by a 20th, so its kind of a requirement unless you're high level or just reckless
The background really comes from the research required to find the truename; this is all leafing through musty tomes, deciphering the hieroglyphs on ancient tombs etc. All the good research that your PCs should be doing when working out either who the villain is, or how to defeat them...
For the good guys it can work the other way around; telling your ally your truename is a sign of great trust etc.
Essentially it all runs back to the underlying myths around these things, and there are various around. It all stems from stories like that of Rumplestiltskin, wherein knowledge of names gives power over the fey (or whatever, depending on the story). There are other myths that cover things like people always hearing if their truename is called aloud. Its also very similar to the concept related to witchcraft/witchdoctors etc. where hair or nail clippings or the like are either required or grant greater effect in working spells on people.
I seem to vaguely recall one concept of magic (from a book i think, rather than a game) wherein all of magic was based on truenames; everything had a truename and knowing it gave you power to create or destroy that item. (i recall that the term was to make or unmake, and unmaking things was ABadThing...)
So, in short, look around for myths regarding true names, or work with the ones you already know, and work them into your game. Rules that might be related are...
Require the name to cast some spells (summoning on powerful entities?)
Grant bonuses to some spells, or lower saves against those spells (enchantments and dominations, but also the banishing type of spells)
Maybe a bonus to dispel magic if you can identify and name the caster? Same for dispel evil if you identify and name the target?
Bonuses for scrying someone, ability to overcome nondetection type effects? (personally i'd use the truename as a requirement to scry by name alone, but thats just me...)