You can get one for under £50 if you google.
L-brackets are about speed, ease of use and accuracy of working, particularly with close distance subjects, and stability.
For example, frame up a table-top still life or product shot, with a normal QR plate in horizontal. Now to change that to vertical, you need to flip the head, adjust the height and shift the tripod to the right to get the lens back in the same position. The head will now have maximum tension on it, making fine adjustments difficult and the tripod will be off-balance. It will take you a couple of minutes to do that carefully and accurately, with a L-bracket it takes a couple of seconds and the tripod will stay properly balanced.