Defrag just puts everything where it should be,,,similar to books in a Library in right place, right order.
Often if Windows are badly fragmented Windows can't find something it needs! Like the drive to touchpad for example Hence minor problems!
I defrag once a week....
Sorry, but this is not correct. Windows will not lose track of a file should it become fragmented on the disk.
Using your library analogy, a defragmented disk would be similar to having a set of encyclopedias stored on different shelves all over the library - the index still knows exactly where all the volumes are but the librarian has to run around the shelves collecting all the individual volumes. This obviously adds time to the data fetching and can slow the machine although unless it is
very defragmented and/or you have a very slow drive installed, it is not noticeable to the majority of users. A library "defrag" would put all the volumes for the same encyclopedia back on the same shelf.
Yes Windows can get corrupt, but this is not a consequence of or solved by a defrag. Switching off a PC/laptop during a defrag can cause problems.
And if your PC/laptop has a solid state drive (SSD) then these should not be defragged at all.