A lot of folks go to their favorite search engine and type in "my city real estate" when the are home hunting - they don't necessarily go to Realtor.com (especially when they are just beginning to look).
So, if the sites they find in their search have IDX based listings available on them (or even a link to the local listings on Realtor.com)- then they will look at the listings through whatever sites they find in the search engines.
IDX would be a benefit to the Realtor who has it on their site more than to the visitors - just because IDX increases the likelyhood of that Realtor being called instead of the listing agent or an advertiser.
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