It a pure turbojet thrust is almost directly proportional to fuel flow. Which would imply idle thrust is around 5-8% of max thrust for most engines. For a turbofan, the low end thrust is proportionally less, so the cite above of 3-4% would be a good ballpark for any modern airline-sized jet engine.
As explained by others, idle thrust on all engines is usually not enough to get moving unless we’re real light weight, have them all running, and are starting off downwind with a bit of slope.
But once taxing, idle thrust on all engines is usually enough to slowly accelerate to too high a speed for safe taxi. So we periodically apply brakes back down to a slow speed and then let the speed build up again. Riding the brakes leads to shortened brake life and at the limit case, overheated brakes.
We want cool brakes at the start of takeoff since our ability to abort the takeoff successfully depends on their total energy-absorbing capability. If we start a takeoff with well-heated brakes and have to abort at max abort speed, the brakes may become “full” of energy before we get down to zero speed. At that point they fade to ineffectiveness and we roll off the end of the runway unable to influence our speed. At 20 mph that’d be embarrassing. At 75mph that might be real hard on the aircraft and the roads and cars and buildings just past the end of the airport.
We often taxi out (or in) on fewer than all engines. It saves fuel and it also saves brake wear. The countervailing point is that to get moving on fewer than all engines takes more thrust from the one(s) you do have running. Exhaust velocity, and hence the blast damage radius behind the aircraft, is more than proportional to thrust. You’re much more likely to blow over equipment or blow in building windows when pulling away with less than all engines running.
Finally, as to Concorde… They were very fuel limited. And they had engines that were very thirsty on the ground. So unlike more typical airliners, they simply could not load an extra hour’s taxi fuel on board to allow for takeoff queue congestion at JFK at peak departure times.
As well their abort scenarios were even more of a knife-edge energy balance than more typical airliners. Brake energy required goes up at the square of the speed whereat you abort. And they had abort speeds 150% of those of more typical airliners. So they really needed cool brakes & tires to start.