Training develops capability. Certification is the evidence of it, and in many trades it is a condition of doing the work at all.
Where it is not optional
Licensed trades require specific qualifications to perform particular work, and the requirements sit with both the individual and the business — see licences and permits.
Certifications expire. Work performed on a lapsed certification can be unlawful, uninsured, and grounds for refusing payment, and the lapse is usually discovered after the work.
Tracking expiry dates centrally, with warning ahead of them, is the control. Held in individual memory it fails eventually — see employee documents.
Where it is a business decision
Manufacturer certifications that permit the business to install or warrant particular equipment, frequently a condition of the manufacturer's own warranty.
Skills that widen what the business can take on without hiring.
The return
Fewer callbacks, because most callbacks are diagnostic rather than manual failures.
Higher utilisation, because a capable technician finishes in one visit.
Retention. Training is consistently among the reasons people stay, and its absence among the reasons they leave for a competitor who offers it — see employee retention.
The objection
That trained people leave. They do, sometimes. Untrained people also leave, and while they stay they produce the callbacks. The comparison is against the cost of not training, not against a person who stays forever.
Paying for it
Training costs are ordinary business expenses. Agreements requiring repayment if someone leaves shortly after are used in some trades and are regulated in some jurisdictions, so they are worth confirming locally before being relied on.