<PERSON> keeps bumping into <PERSON> for several reasons throughout the show :
1. When <PERSON> needs a partner who can cook with him and help him make money (Season 1 & 2)
2. When <PERSON> wants to make sure that his identity remains confidential. So even when <PERSON> is working solo, he again brings him with him, so that <PERSON> does not get caught and hence does not spill the beans. (season 3)
3. <PERSON> constantly manipulates <PERSON> for achieving his own goals. He knows how often he has successfully manipulated <PERSON> and made him do things which <PERSON> wanted eg. Killing <PERSON>, Killing <PERSON>, etc. (season 4)
4. After <PERSON> being gone, <PERSON> again needs <PERSON> to assist him on his empire business
5. With <PERSON>, not being very fond of <PERSON>, <PERSON> acts a mediator between them. 725
<PERSON> needs <PERSON> as well to run his business after <PERSON>.
6. Later in season 5, <PERSON> acts as the primary antagonist for <PERSON> and also becomes a very important partner for <PERSON> in order to nab <PERSON>.
7. Finally <PERSON> again comes into picture, when <PERSON> is on his road to redemption, where he tries to correct all the wrong that he has done to <PERSON>.
So <PERSON> was always an integral part of the story. He is kind of 2nd lead in Breaking Bad.
[EDIT] After the end of <PERSON>, how exactly <PERSON> needs <PERSON> to establish his empire? Why couldn't he simply part ways with him ?
<PERSON> can't do it on his own. He definitely needs more people. <PERSON> is extremely resourceful and also provides the muscle, but it is highly unlikely that <PERSON> would work alone for <PERSON>, because <PERSON> hates <PERSON>. Remember that after the death of <PERSON>, <PERSON> was infuriated and it was <PERSON> who calmed him down. Moreover why should <PERSON> involve new people in his business ? <PERSON> already knows his identity and knows to cook crystal meth as well. Additionally <PERSON> knows this very well that he can manipulate <PERSON> whenever he wants, because he has successfully done this so many times. So <PERSON> just can't get rid of <PERSON>. <PERSON> is the best resource he can have. 190