Imps are the lowest type of spirit, and are generally quite rude, thus only performing well as a messenger, or in large numbers of them. Imps are commonly also trapped in reflective surfaces to create scrying glasses.
Like all spirits, imps possess some form of magic, but it is extremely weak compared to the others. Unlike other demons, they are unable to change form and thus gain respite from the pain the Earth's pull causes on their essence. As the lowest type of spirit, they are easily summoned by even mildly-competent magicians in large numbers.
Nathaniel used an imp that took the form of baby in his personal scrying glass. The only other imp mention in slight detail was in the 1st book; he was carrying mail from Lovelace when Bartimaeus detained him and proceeded to go through the said mail whilst forcing the imp to call him by a different glorifying title everytime he was addressed.