Customer service is the handling of everything around the work: the enquiry, the arrangements, the arrival, and what happens afterwards.
Why it carries more weight than the work
A customer generally cannot assess the technical quality of what was done. They can assess whether the phone was answered, whether the visit happened when it was promised, and whether they were told what was going on.
So those become the basis of the judgement, and of the review and the referral.
The three that matter most in a service business
Answering. An unanswered call is usually a lost job, because the caller moves to the next number rather than waiting.
Arriving when promised. Or telling the customer before the window passes rather than after. Lateness communicated in advance is tolerated; lateness discovered by waiting is not.
Explaining. What was found, what was done, what to expect. Its absence is the most common cause of a dispute about an invoice that is entirely correct.
Setting a standard
The same requirement as quality control: stated, so it can be taught and met by someone other than the owner.
How quickly calls are answered and returned, how appointments are confirmed, what a customer is told at completion.
Recovering a bad experience
A problem handled well produces a more loyal customer than one that never occurred — see complaint handling.
Its financial effect
Retained customers, higher lifetime value, and referrals, which are the cheapest source of work available. It is not a soft cost; it is a marketing channel.