Alright, I am going to explain this one piece at a time; first, you need to understand the category that all of these fall under: thoughtforms. A thoughtform is anything that existence is based either entirely or partially on someone's belief in it. A Servitor is more near the "entirely" end of the spectrum, with a Familiar being in the middle, and an Egregore being closer to the partially/none side. A Servitor is a creation of a magician that serves a specific purpose. For example, a Servitor created for finding lost luggage would be used for exactly that purpose an no others. A Familiar, in my opinion, is more like an imaginary friend. You use your imagination to create the Familiar's shape, movement habits, and personality but, after you have created it, it seems to take on a life of its own; doing things that fit its personality but that you have not requested (nor, at times, wanted) it to do. An Egregore, on the other hand is something created, usually by a cult or group as a Servitor, that is believed in by multiple people. Unlike the other thoughtforms, is usually worshipped in a minor way to convince it to do one's bidding. Although Egregores can be incredibly powerful, they tend to become quite difficult to work with if you do not set them right in the first place. For example, you make an Egregore and explain its personality and look to the gorup you are in; several of the members mentally relate the description you gave them to the Greek God of War: Aries. Now you're screwed; you have just created an Egregore that now an abrupt add on to its personality because of other peoples views. You now have a blood-thirsty warmongering Egregore; nice job (if you couldn't tell, that was sarcasm). Now, not all Egregores are created intentionally. For example, there is a gun-egregore; if you have ever held a gun in your hand and had the urge to shoot something or someone, then you have felt how this egregore effects people. This Egregore was created from society's belief in guns as violent and merderous and has been around at least a hundred years, therefore it is quite a strong example of an egregore (this is why I strongly recommend that all practicing or non-practicing magicians keep guns out of their homes at all times). Another less violent one mentioned in the book "Modern Magick" by Patrick Dunn is the parking lot Egregore. The author has seen this egregore before as a small ten-year-old child with a blue windbreaker and blonde hair; now, if maybe given a small offering (like a few pieces of bread and a glass of milk) this egregore may help you find an open parking place; something that even magick has had trouble with in my experience. Now, the use of as many Servitors as you need as well as a few Familiars to help you out as guides and teachers and even an Egregore or two is, in my opinion, almost a requirement; spells and Rituals can work wonders, but a thoughtform can help in some of those tight situations when an altar just isn't available.
This is the reason why thoughtforms are so abundent, I mean, c'mon, would you really sit down and decorate an altar then preform a long, draw-out ritual in the middle of a bookshop just to find that book you need? Of coarse you wouldn't, you would have a servitor do that for you.