Evolution tinkers with the designs which it already has on hand; It does not have the ability to design parts de novo as a human engineer does when coming up with a new table or canoe paddle shape. Thus, how much mutation (which must occur but is not certain to occur), subject to natural selection, can alter a structure is a major limit on the kinds of structures which can form in living things. This in turn limits the possible functions which those organisms can perform.
Work Step by Step
Think about the human engineer mentioned in the problem. If they are asked to design a sailboat but limited to starting with existing designs for kitchen counters, then also limited to making random alterations in the counter and testing each of those new designs as a sailboat, the engineer would have a long and frustrating job. Especially at first, many clunky designs would be produced. The most perfect possible sailboat would be unlikely to arise from these limitations, but a very functional sailboat could be made with some experimentation.