A Clustered Index Scan is similar to a Table Scan. IE: The data is being read row-by-row
A SCAN happens when the optimizer determines that 1) all/most rows need to be returned (so it would be a waste of time reading the index keys), 2) the index is not selective enough, and the optimizer thinks it needs to read all/most of the index. 3) the index stats are found to be out-of-date 4) when the query adds functions to a column’s data, obscuring the columns data from the optimizer.
An (index) SCAN suggests that because a normally sufficient index is NOT sufficient at the moment – more data may be being returned than (normally) needed – suggesting more filtering may be needed – perhaps in the WHERE clause.
A CLUSTERED index seek is beneficial because the data can be read from the index, without having to look at the table.