Why is the clearance delivery frequency included on an approach plate?
The FAA Aeronautical Chart User’s Guide: Terminal Procedure Publications in section “Bottom Briefing Strip (Communications Information),” page 15 in the revision effective 29 March 2018, explains:

The communications briefing strip contains communication information when available, in separate boxes, listed from left to right in the order that they would be used during arrival …

Clearance Delivery (CLNC DEL) frequencies; where a Control Tower does not exist or is part-time, a remoted CLNC DEL may be listed …


Relevant to the briefing strip in the question, this section adds

Note: Part-time operations will be annotated with a star. Check Chart Supplement for times of operation.

According to the Chart Supplement for McClellan-Palomar, the tower is indeed part time.

AIRPORT REMARKS: Attended 1500–0600Z‡ …
COMMUNICATIONS: …
    TOWER 118.6 (1500–0600Z‡)

Other use cases when having the clearance delivery on the approach plate is convenient are obtaining a new clearance during a brief stop and closing an IFR flight plan through clearance delivery, either local or remote, when the tower is closed.