One of the reasons is to do with listing priority (I'm assuming that you have checked out your competitors and you are cheaper/better - for fixed price ads)
A lot of people searching on eBay would just accept the standard "best match".
The way I read "best match" is that the folks nearer the top of the list (not the paid adverts, right at the top) are the adverts that most closely match eBay policy, and whose sellers have being giving high quality service.
e.g. check your photos are clear, of the correct size, no text or extra graphics (other than faint watermark), not generic photos (unless brand new item), and so on.
They also prefer fixed price with free post, 1 day handling, and a 30 returns policy.
Check the policies and see if you can tick all the boxes. they also provide a program for bulk changes to adverts
So this will increase the ranking of your ad and make it more likely to get seen.