Dinettes used to be a more popular configuration in the '60s/'70s/early-'80s. They were the standard salon configuration on popular boats like the Westsail 32 and Peterson 44, among others. You don't see them too often on production boats anymore.
Think of a dinette much like a diner booth. They are handy for young families, who are always looking for nooks where the kids can sit out of the traffic pattern and do homework, arts and crafts, etc. It also makes it easier for parents to "manage" the meals with young kids -- generally everything is within easy reach.
But that "easy reach" is also one of the downsides -- they tend to feel a bit cramped, especially on small-mid-size boats. Also, ingress/egress for the outboard seating positions is not the easiest.
My preference is when the dinette can be turned into a U-shaped, wrap-around bench seat. This gives everyone a bit more elbow room, at the expense of some lost table surface area. But in either configuration, it is a nice advantage not having to always include the opposite settee in the seating configuration -- freeing the path fore and aft, as well as the opposing settee.
Here is the dinette from a Peterson 44, for those unfamiliar with the configuration: