Follow up contacts are most important in establishing a new client relationship.  It shows that prospective client that you are interested in them, and their business, and puts your company a level or two higher in their 'imaginary tier level' of someone to do business with.  Even better, that short; 2-3 day at best, lapse you waited to make that follow-up contact may have opened up opportunity for you/them that did not exist when you made the initial contact.  So instead of just a simple follow-up call, you have now secured a real business contact/CONTRACT and acquired new business for your company.  Simply because you kept your word, and followed through, it's those little things that encourage people to do business with you instead of your competitor.