I can think of three sayings that overlap to some extent but are distinctive in their own ways:
If it ain't broke, don't fix it.
The perfect is the enemy of the good.
Don't reinvent the wheel.
All three of these expressions promote the idea of functionality or good-enough-ness as the primary goal to focus on in completing (or deciding whether to take up) a project. In particular, they argue for a pragmatic assessment of effort required versus benefit gained in a particular undertaking. All three sayings express a fundamentally cautious, conservative, and perhaps anti-innovative (or at least innovation-agnostic) approach to the status quo.
"If it ain't broke, don't fix it" amounts to saying "Leave well enough alone." That is, don't change something that already does the job adequately, just for the sake of changing it or in hopes of improving it marginally. As a matter of resource allocation, this makes sense—because if you throw your resources into a project that will at best improve something that already works satisfactorily, you may be missing the opportunity to undertake a different project addressing a genuine problem that, if solved, would yield a much greater return on the investment in time, effort, and money.
"The perfect is the enemy of the good" emphasizes the diminishing returns involved in trying to upgrade something that is not just functional but good to an impossibly high level of excellence. It warns against, in effect, treating a success (achieving something good) as a failure (because it isn't perfect)—because if you continually reject the good thing in the name of serving the (imagined) perfect thing, you may never enjoy the benefits of the good and never achieve the perfect.
"Don't reinvent the wheel" stresses the idea of building on prior efforts in a project to improve something, rather than going back to the beginning of the technology or process and starting from scratch. The point here is that certain established practices, techniques, or insights are useful and ought not be discarded even if the existing product or process needs considerable improvement.
So we have a saying that warns against meddling with a successful process or product ("If it ain't broke, don't fix it"), a saying that warns of the law of diminishing returns with regard to efforts to transform something useful and functional into something flawless ("The perfect is the enemy of the good"), and a saying that warns against too radical a rejection of past achievements in trying to improve on them ("Don't reinvent the wheel").
