A lot of architecture uses symmetry. It is most important in the Classical style of architecture, based on the ancient buildings of Greece and Rome. The fronts of many Welsh chapels are symmetrical because they were inspired by Classical architecture. Chapels were built in other styles too. Some were so simple that they didn’t really have a named style, and some were Gothic.
If you imagine a line down the middle and the building looks the same on both sides, the architecture is probably from the Classical ‘family’ of styles, such as Greek, Roman, Renaissance or Italianate. Capel Peniel at Tremadog is a good example. It was built in 1810. The columns and wheel window were part of original design but not built until 1849. If you look at the plan of the building, it is symmetrical in layout too. You can find more information about Capel Peniel, including a 3-d fly-through, on the website of Addoldai Cymru.