In short, the answer is that over the phone interpreting can be easier than on-site. If you have a consistent need of less than two hours at a time, OPI is a great option.
On-site interpreters typically charge a minimum of two hours at a time (which can be extremely inconvenient for scenarios like routine check-ups). It is also typical for the requestor to pay any travel fees for the interpreter, which can be cost-prohibitive for rural areas.
Another factor to consider when comparing the two modes is the diffusion of the language. If, for example, an indigenous language of lesser diffusion is required, then getting an on-site interpreter could mean a logistical nightmare.
“Except for in certain cases like having a staffed interpreter for a full-time need, OPI is usually a simpler, more cost-effective solution for interpretation requests.”