Well, be specific about what indexing is: it layers a B-tree on top of what programmers would think of as an array structure. Indexes are not just something which is always faster than a sequential scan in exchange for slightly slower writes - there are important cases where the index is slower than just doing a sequential scan. Obviously you hope the query planner will avoid using an index in these cases, but the important thing to bear in mind is that indexes are not magic go-faster stripes :)

The problem happens when this B-tree is too large to fit into your working memory: swapping parts of the B-tree in and out of memory repeatedly to answer a query is very slow. The solution that partitioning offers is to allow you to split your indexes (and your data) so that, depending on your access-patterns you could see less swapping of that index.