You capture more reviews.
On average, people tend to react and respond to advertisements (direct mail, door hangers, or yard signs), emails, and other marketing channels after they've seen them 3 - 4 times. People get busy and forget.
If you only ask or tell prospective customers about your service once, you dismiss the additional opportunities you have to get reviews/business.
Keep your brand top of mind with multiple impressions, more reviews, and follow-ups.