There are several reasons that causes problems in the in the window registry, it can get problem when some uninstallers for windows software do not remove all the traces of the software from the registry, other possible problems could be orphaned start-up programs or missing or corrupt hardware drivers and so on.
These problems increase the registry size and slow down the computer. When you operate the computer the existing problems will make windows take more time to load, search and read data from the registry. Moreover this problem will lead to windows crash and error messages. To registry cleaners mend up this problem by scanning the registry, it picks out unnecessary or damaged pieces and either deletes or repairs them.
The process of cleaning can be done manually but the large size of the registry database filled with debris and invalid entries makes it practically impossible to do it manually. This is registry cleaners are put to use. The registry cleaners automate the process of spotting for invalid entries or broken links and removing them from the registry.