Sangkhalok Museum focuses on the ceramics that once helped make the Sukhothai region famous across trade routes. The collection is specialised, but that is exactly why it is useful: after seeing the temples, you get a more domestic and commercial angle on the old kingdom.
It is best for travelers who already like pottery, craft history, or small private museums. If your time in Sukhothai is tight, see the historical park first; if you have an extra hour, this adds texture to the story.