A site visit is going to the job to look at it before you put a price on it. It lets you measure rather than trust the customer's numbers, see what you will be working on and around, check access for people, materials and machines, find the services you will connect to, and spot the conditions that change the price: a wall that is not what the customer thinks, a slope, a switchboard with no room, a roof you cannot safely reach. It also lets the customer meet you, which matters more the larger the job.
Not every job needs one. Standard work you have done many times can often be quoted from a phone call and a photo, with the assumptions written into the quote. Measured jobs, hidden work and anything large usually need a visit. The guide on quoting over the phone versus on site sets out how to decide.
In practice
A site visit costs real money, and it is worth knowing how much. Take a trade whose time costs $100 an hour to the business. A visit with a 40-minute drive each way and 30 minutes on site takes 1 hour 50 minutes, so about $183 of time before fuel. If one in three visited quotes turns into a job, each job won is carrying about $550 of quoting visits. That figure has to be in the prices somewhere, or the visits are being paid for out of margin.
There are three ways to handle it. Build the cost of visits into your prices on larger work, where the visit is expected and the job can carry it. Charge for the visit on small jobs, often as a call-out fee credited if the work goes ahead. Or reduce the number of wasted visits by qualifying the job on the phone first: ask for photos, a rough budget and when they want it done, and send a ballpark before you drive out.
Make each visit count. Photograph and measure everything that drives the price, check access, and build the quote before you leave the street, while the details are fresh and the customer is still thinking about you.