Hi - very briefly, aircraft will always prefer to take-off and land into the wind. If an aircraft has an approach airspeed of 120kt, in still air its groundspeed will also be 120kt. If it's flying into a 20kt wind, it will still fly its required airspeed of 120kt (remember it's the speed of the air flowing over the wings which is important), but its groundspeed will only be 100kt. This has a number of advantages:
1. The momentum of the aircraft as it touches down is much, much less - imagine the energy of a 100 ton object moving at 100kt vs 120kt - it's much less energy which the brakes and tyres have to stop.
2. If the aircraft is flying down a 3-degree glidepath, its vertical speed reduces as the forward speed does, in the example above it is 120*(sin3) vs 100*(sin3) or 6.28kt vs 5.23kt. Again, about 20% less vertical energy to manage.
But...if it now has a 20kt tailwind, its groundspeed becomes 140kt and so there is much, much more vertical and forward energy to manage - this makes the landing roll much longer and might mean the aircraft can't stop safely on the runway length.
On a dry runway, aircraft can cope with a few knots of tailwind. On a wet or snowy runway, when braking performance is reduced, there is much less ability to accept a tailwind.
Your example of replacing 12/30 with another 06/24 is very much a business decision by the airport - there will be a few occasions where the wind is strong and across 06/24, where 12/30 would have been better for the aircraft. The airport has to decide whether it's better to accept generally higher capacity with two parallel runways, at the risk of occasionally having crosswinds which stop some aircraft using 06/24. Such decisions will usually have very detailed analysis of prevailing winds, likely aircraft types using the airport [bigger, heavier aircraft might be able to accept more crosswind than smaller types], and so on.