These days, most calls are what’s called automated or reservationless conferencing. Meaning you can have a call at any time 24/7 with no setup required. Sometimes, though, it’s nice to have an operator involved. It might be because you’ve got a large number of participants. Generally if you have over 50 people, it’ll be much more effective to have an operator involved to eliminate background noise or to help conduct the call for you.
Other reasons you may want to have an operator is if you’re collecting information. For example, in a sales call or a marketing type of webinar. Operators can be involved to collect people’s name, company, or email for you. The traditional type of operator-assisted calls is what’s called an IR call or investor relations call. These are typically done only by public companies on a quarterly basis when they report their financials.