aad:
Does that apply to Chinese dramas too or just Taiwanese dramas?
Chinese dramas get dubbed for a few reasons:
1.) Creating a uniform accent/dialect. Mainland China has a lot of dialects, so they often dub in order to make the character's dialect match the region they are supposed to come from and/or the other characters from there as well.
2.) Sound quality. A lot of Mainland China productions (especially historicals) are filmed in the same locations, some with several filming projects going on at the same time. It makes it quite noisy. So they do the filming, and then they go in afterwards with dubbing (this is also why you'll sometimes find dubbed titles where the actor is doing their own dubbing (which makes people who dont know either doubt that it's their voice, or ask what was the point)).
3.) To make a character have a certain vocal "quality" that matches the character (mainly applies to fantasy historicals and wuxia/xianxia/xuanhuan).
4.) (Hypothesis from observing): Censorship. It's not uncommon for censorship to come in and say that certain lines need to be axed or changed (there's literal lip-reading videos all over social media to determine what characters are actually saying underneath the dubbing). Since the actors tend to have such a jam-packed schedule with other filming, appearances, shoots, promotions, sponsorships, etc, it's easier to have a voice actor on retainer to come in whenever needed. So then the voice actor would need to have dubbed the entire production in order for that fix not to be as noticeable.
Taiwan doesnt have these problems and/or to the same degree. So you wont typically find dubbing in Taiwanese dramas, especially recent ones. With the exception of actors/actresses who natively speak a different language, and so they dub them to make them more understandable or to compensate for their lack of knowledge of the language.