If you don’t have a clustered index on a table, you can have serious problems with empty pages that never get de-allocated. I once found a heap table that had 125 small rows, but was using 16 GB of storage. Performance was horrible when the query had to scan the table.

Another reason to have a clustered index is to reduce the space used by the table. If you don’t have a clustered index, each index will usually consume more space.