Aryan - appendices are 'supporting' information and documents that are not needed in the main text of a thesis and, therefore, not included in the main text word count. They are often structural i.e. ethics documents/approval, a more detailed account of the literature review search process (beyond the summary information in chapter text. Some people make the mistake of adding 'anything' in their appendices - either to restrict the word count/and or to impress with the breadth of reading. Remember that examiners do not have to refer to the appendices - or at least not in detail - so 'vital' information to the thesis as a whole should not be included. If the appendices are larger than the main body - it's a warning sign that perhaps too much has been used - and that quality (not quantity) is important.
I would go even further - and say to my PhD students - If it is important it should be in the main text, and if its is not it should not be there! It is of course different if for example 'data' in its many forms has been generated in the project or software code has been written , but then I would suggest that such stuff is placed in a digital archive so that it can be accessed and used easily and properly, with a clear description and link in the main text. I would not put results there. Save paper; less is more.
This might happen with systematic reviews, especially where the PhD has involved more than one systematic reviews, and the results tables are located in the appendices.
That should really be an exception, indeed, a very rare exception. As already pointed out by Dean and Kelvyn, if what is important is put in the main text and what contributes to the study but not to the point of inclusion in the main text is put in the appendix section, the appendix shouldn't be larger than the main text.