Blanking it out basically makes it so it will search all posts within the admin defined range. The limit is there since on sites with tons of posts searching every post can be a pretty large database hit. Now that we're fulltext indexed it might not be quite as bad, but when a user searches something like 700,000 posts for the word 'truck' it can be fairly intensive
This is why we keep the default a smaller range, just so users don't bombard the search engine unless they really need to do extensive searching.
