The act opening an arena show has usually been placed there through a negotiation that has little to do with musical suitability. The support slot is a tradeable asset with a market price.
The exposure is the product being sold
An opening act plays to an audience many times larger than it could gather alone, night after night, across an entire territory.
That access has obvious value, and anything valuable that is scarce ends up being priced. Support slots are allocated through negotiation because demand exceeds supply.
The headline act controls the asset, and its representatives decide who receives it and on what terms. In practice that decision is made by an agent and a manager rather than by the performer.
Money can move in either direction
Established support acts are paid a fee, though usually a modest one relative to what they would earn headlining smaller rooms.
Developing acts frequently pay for the position instead, directly or through their label covering costs. The arrangement is a marketing expense rather than an income.
Which direction the money flows depends entirely on relative bargaining power, and the same act can be on either side within a few years.
Labels use the slot as a development tool
A record company with an artist to establish will pursue support positions on tours reaching the right audience, treating it as promotion with a measurable cost.
Where the headline act shares a label or management company, the placement is easier to arrange and the terms are internal.
This is why support bills so often connect back to the same organisations, which audiences read as a coincidence of taste.
Conditions are set by the headline production
Support acts work under real constraints: restricted sound, limited lighting, a fixed short set, and no access to parts of the stage.
These exist to protect the main performance and to keep changeovers fast, and they are written into the agreement rather than negotiated nightly.
An opening act that appears underpowered is often performing exactly within the limits it agreed to accept in exchange for the position.
The audience arrives late by design
Venues schedule support early, when a substantial part of the crowd is still outside, in queues or at bars.
That timing suits the headline act, whose audience should be present and settled for the main performance, and suits venue revenue.
It does not suit the support act, which is why acts with leverage negotiate set times as carefully as fees. A later slot in front of a full room is worth more than a longer one in front of an empty floor.