Why do we need Troi and Guinan?

Counselor Troi was an official member of Starfleet and had been trained in the Psychology division (Science and medicine), whereas Guinan was a civilian and hired to serve drinks at the bar in the mess hall. While Guinan did provide good insight into things even Counselor Troi couldn't help with, she couldn't be around consistently (whether it be her character, or the actress's availability). Counselor Troi, on the other hand, was a regular member of the crew, worked regular hours, and could be consistently relied on for missions or helping crew members with psychological problems. She could also act as a diplomatic aide. Her ability to read emotions and sometimes even thoughts helped a lot. On the other hand, she was not as old as Guinan, so Guinan had insight and maturity that Troi might have lacked. Counselor Troi was also limited by certain rules of her job, whereas people could meet with Guinan in an unofficial capacity.

Looking at it from a real world viewer perspective, we didn't technically need Guinan, because like you said, she served a similar role to Counselor Troi. However, Guinan was not a main character, and was not the designated sex symbol/telepath of the group. She was a re-occuring character with good development. Chances were, Whoopie Goldberg wasn't always available to be on the show, probably had other things going on in her life at the time, and probably didn't want a full-time job in that area. Marina Sirtis, on the other hand, was available, wanted to do the job, and was good at it.

I loved them both.

And every episodes that featured Troi and her mother was pure GOLD!