Usually, it's recommended to get a rest day in just for the sake of letting your body have some recovery with minimal overall stress put on it. Though, many high-level athletes such as olympic lifters don't often take "rest days", rather, they do active recovery, where they'll take 1-2 days per week that are spent doing other things like taking a light jog, swimming, or other activity that they don't do all-out, but still don't tax the muscles in the same way.
Of course, when you get older, rest becomes far more important. In my early to mid 20s, I could train for 90 minutes and run 3-6 miles every day and never get worn out by it even if I never took a break. Now, nearing 40, I can't train more than 2 days in a row before my body tells me to take a day off. I suppose you don't NEED to rest up if you don't want to, but you never know, it might help your progress even more to let your whole body recover one or two days each week!
"A 'hardgainer' is merely someone who hasn't bothered to try enough different training methods to learn what is actually right for their own damned body." - anonymous