There can be many reasons why legalization is necessary. Basically, everything that is firmly connected to the ground is subject to approval.
For example, if your house was built by one or more previous owners, you may at some point discover that part of it is not "legal". You can see that in the cadastral extract, or in the Escritura - in which only a shed is described - although there is a very stately house there.
In this case, it is advisable to legalize the situation, and generally to comply with all obligations. An application for legalization is roughly equivalent to a building application. This will avoid nasty surprises in the future. Or you want to expand a house that is not yet in the public records as it is - then you would have to submit a legalization / extension application.
We have created a large number of legalization applications over the last few decades, relieving our customers of considerable worries.
Do you have any questions about it? We are happy to help!