It makes an interesting review to know an actors previous roles because it shows the range of his acting abilities. Playing a variety of roles may be a surprise for some fans since the disguises and costumes and accents can sometimes prevent you from knowing the actor's identity. Actors are sometimes not listed for credits on movies so it is difficult to know all their roles.
Sure, it's fun to know about an actor's past roles. It's not a necessity for me, but it's interesting when you can find a little tidbit about what they have done in the past, especially if that is very different from what they are doing now. Old commercials are especially entertaining!
Seeing your favorite actor in a role long ago and seeing how much they have changed is always fun. I would love to see clips of some of my favorites in their earlier days. It would be fun to see what they used to look like and how much different they are now.
Sometimes knowing about actors previous roles makes us understand where they coming from in their acting career and the growth thereof. Most actors start by playing small roles such extras and in due time they take major roles and even more importantly fall into a certain characterization which they perfect.