For undergraduates, your dissertation is an extended research project which contributes significantly towards the final assessment for your degree, and is often a key indicator of your abilities as a student and researcher.
For Masters and doctoral students, your thesis is the central impetus of your research.
Before you begin such a significant piece of research at any level, it is vitally important to see what else has been written on your topic. If you are starting doctoral research it is important to establish that no one else has already carried out the specific research that you intend to do. You will therefore need to look at previous theses in your field of study.
Theses are also extremely valuable sources of information in your studies generally, as they consist of substantial primary research in specialised topics and provide very detailed data and analysis. They may be so specialised that they provide information unavailable elsewhere, and their bibliographies are usually a very useful source of wider reading on a topic.