The benefit of getting a rod and reel combo is that you will know that the specifications of each will work well with each other. You won’t have to worry about whether the flexibility of the rod will be compatible with the gear ratio of the reel, because a professional has already put in the work for you in figuring that out.
The combo also has the additional benefit of cost. When packaged together, you can often find combos that are priced at a very reasonable rate, usually cheaper than buying the two components separately. Therefore, if you’re on a budget you can get a little bit better quality equipment within your price range if you go for a combo set.