As we know, the index is one of the most powerful tools for optimizing database performance. As you insert, update, and delete rows, indexes become more fragmented and less effective. Because of this, DBAs regularly do index rebuild operations.
With the growing sizes of databases, index rebuilds can take a very long time. Combine that with the business needs for your applications to be always available and performant and this can be an issue. Big OLTP environments with busy workloads often have very short maintenance windows with some too short to execute large index rebuild operations.
Read more about it in the Azure blog.