What does the custom plants feature provide you? Well...:
Create custom plants using existing plants as a template, or from a planting row/square on your plot, where it will copy the planting schedule and spacings you set.
Add your own growing instructions/notes, plant family and default plant spacings.
Customise the planting schedules for your custom plants so that they match when you actually sow, plant and harvest them each year.
Filter your custom plants on the plot's plant menu so that you can easily find and add them to your plans.
Custom Plants will throw Crop Rotation warnings just like standard plants based on the plant Family you choose.