Some people want a specific username (whether it be an internet site, email address, etc), and that name has already been used. So if they really want that name, they will add a number at the end. My username, for example, is just my old University username. They give it to us (first initial and last name). If that username already exists, they put 2, 3, 4, etc at the end. So that explains my username. 

Mr. Zed : They put numbers in usernames and passwords so their account can be a lot more secure..
I don't put numbers in my name because I don't like too. :p

Well for me it is my Xbox Live and PSN username. tgags was taken, so I added 123 to the end of it. When I signed up for Vizzed, I put 123 so it would match.

For my username number 122, this was the number I first used on DeviantART, the number was originally going to be 121, but when I joined on that site before they did that name change thing, I was going with #121 because 121 was the state route number I used to live on before I moved in 2012, but it was taken, so I went one number higher which was #122, but I found out, I liked 122 better than 121, so that's pretty much the origin of why my number is 122

The number(s) at the end of some usernames can be because the username that they chose are already in use or their username are too short so they put numbers on their username. I somehow also don't like numbers on my username, but if my username (Zed) is already in use, I simply add another letter or two in it. Like what I did to my name. Sometimes, I add the number 09 on it because it's my birthday (The 9th Of December). I also use Zed09 sometimes. But it depends.

Now, most people probably put numbers at the end of their username if that name was already taken. For example, let's say somebody wanted the username Bob, but it was already taken. So then they would try putting numbers at the end to make it Bob007. Usually, people just put random or common numbers with no significance whatsoever. In this case, let's say this person is a big James Bond fan.
Other people might just have numbers at the end because it's their trademark name. Take my username for example. Ever since three or four years ago, I've been using the username SetoKaiba1025. I use the name because... I have to think about this one. Um... Oh yeah! That's because October 25th is Seto Kaiba's birthday (and also my older brother's birthday). The name SetoKaiba wasn't taken on that site, but I put numbers at the end anyway. There might be some things that I haven;t said, but that's all the knowledge I have about this topic.

For me, I don't really have a real reason. Again, they are just usernames, lol. I guess the number 3 at the end of the username makes it look more complete and cool because "mohammedroxx" just feels a bit incomplete for me. lol, don't know what else to say.

They do this to make it original. I didn't do it because my name is Barathemos. Self explanatory.

I try to not add numbers to the ends of my user names because then I can say that they are my own original identities online. When they are taken in all forms (I have quite a few) or not enough characters are available, I normally add 493 to the end, since that is the number of Pokémon I was able to memorize back when I was really into it (don't ask me why, I don't know why I did that.)

Probably because they want to. People would like to have their name unique, so they may put numbers at the end of their username. Some may have a favorite number and would want to add it. Some may just want to add it for no reason.