Here is one study that compares linear progreesion versus variable progression in strength training. The first group sought to increase strength over 12 weeks by gradually moving to heavier loads and a lower number of repetitions. The second group changed things every time they lifted. The variable group showed significantly greater gains than the traditional group.
There are plenty of other studies out there. Here's one that compares periodized to non-periodized resistance training over 9 months in women tennis players. The periodized program produced significantly more benefits.