I was asked to review a conference proceedings which topics I'm not entirely interested or knowledgeable (there is an overlapping topic I'm qualified to review though). Since I'm new to the field, I searched the ranking and I found it's close to the bottom tier. The colleague who asked me for reviewing it says it might help gaining experiences since I'm new to the field. Would it be helpful for gaining an experience, or would it be a waste of my time if I'm not interested in publishing in the journal/conference anyway?
"Concerned" is not quite the word I would use, but it seems very reasonable to take the quality of the publication venue into account when deciding whether to accept the referee request.
Most academics I know hit a certain threshold age / level of seniority after which they get more referee requests than they can reasonably take on [here I should say that in my field, mathematics, refereeing one good paper can be a lot of work -- it is no exaggeration to say that doing so may require putting one's own research on hold for several weeks]. So they have to pick and choose which requests to take. They pick papers through a combination of personal interest, sense of their relative expertise (if you feel that you are one of the only people in the world qualified to competently referee a paper in a reasonable amount of time, it's pretty hard to say no) and estimations of quality of the author/editor/venue.
Based on what you said, considerations like the above seem to point in the direction of your declining to referee the paper. If you are so junior that you have almost no refereeing experience at all, then you might want to take it anyway...but then again you still might not. I would encourage you to discuss this with your thesis advisor (if you are a graduate student) or with a faculty mentor. You want someone with intellectual and sociological expertise in the field to weigh in on whether this would be a good use of your time.
Reviewing for a journal/conference will (ideally) not help you publish there (the ones reviewing your papers will not know whether you are a reviewer).
But (as your colleague noted), reviewing for a journal/conference may be a step on the way to reviewing for another journal/conference. And reviewing may be considered positively when your department considers you for promotion and/or raises.