An artist selling shirts at a concert usually keeps only part of the money. The venue takes a negotiated percentage, and the practice follows from how a building recovers its costs.
The ticket does not pay for the building
A venue's costs include staff, security, insurance, utilities, maintenance and debt on the property itself, and these continue whether or not a show is booked.
Much of the ticket price goes to the performer and the promoter, so the venue relies on what happens inside the building on the night.
Concessions and merchandise are therefore not incidental income. They are a substantial part of how a room stays open across a year of uneven bookings.
The venue supplies the selling operation
The house provides counters, lighting, power, payment terminals and the staff who sell, all of which have to exist regardless of who is performing.
It also carries the losses, since staff are paid whether the audience buys or not, and the venue absorbs that on quiet nights.
The percentage is the price of that service, and rates are typically lower where the artist supplies their own sellers and higher where the house does everything. What the house provides is what the house charges for.
Rates vary by item and by leverage
Printed goods usually carry a higher rate than recorded music, and some agreements exclude recordings entirely as a matter of convention.
An artist who can fill the room repeatedly negotiates better terms than one being given a first opportunity, in the way any supplier with alternatives does.
Large productions sometimes bring their own equipment and staff entirely, reducing the venue's role and its share accordingly.
Selling elsewhere changes the arithmetic
Sales made away from the building, through an artist's own channel or a subscription, fall outside the venue agreement.
That has pushed acts toward direct online sales, promoted at the show but completed afterward, which alters what the merchandise table is for.
The table then functions partly as a display and a point of contact rather than the main sales channel it once was.
Merchandise carries the tour's margin
For many touring acts, merchandise contributes a large share of the profit, because production and transport costs are largely fixed against ticket income.
A run that sells well at the counter can be profitable while one that does not is merely covered, on identical attendance.
That is why the percentage is negotiated as carefully as the guarantee, and why it is one of the more contested items in a booking agreement. Several points of merchandise can outweigh the fee itself.