Liquid adjuncts come in the form of sucrose or grain syrups like rice or wheat syrup. Added directly to the wort, they can reduce the loading in the mash and lauter tun (due to being liquid and not solids), thus increasing your kettle's capacity. Moreover, they can also be added after the fermentation process as a type of priming sugar to give the beer a sweeter flavour.
Adjuncts do play a part in imparting flavour to the final brew. But, more than the flavour, they also provide additional features and characteristics to the beer, such as better retention and increased nutritional value.
It is important to note that different types of adjuncts are used to create different types of beer. For instance, candi sugar (a type of solid adjunct) is used to make Belgian ales, while honey (a type of liquid adjunct) is used to create a mead.