Why are segmentation and paging sometimes combined into one scheme?

Segmentation and paging are often combined in order to improve upon each other. Segmented paging is helpful when the page table becomes very big. A large contiguous section of the page table that is unused can be collapsed into a one segment table entry with a page-table address of zero. Paged segmentation handles the case of having very long segments that need a lot of time for allocation. By paging the segments, we decrease wasted memory because of external fragmentation as well as simplify the allocation.