The other modes have some really stupid maps.

Deathmatch would, in theory, be good for practicing mechanics. Trade District is enormous. Most of the time is just spent walking around and looking for enemies. Abyss has so much verticality throughout the whole map that champs without vertical mobility will also need to spend an eternity walking and searching. If someone is trying to do aim warmup, they want the highest volume of interaction per minute.

Onslaught can be a fun mode, but Marauders Port is just a dealbreaker for me. It's such a dumb map. It lacks any form of subtletly. The verticality is so extreme that players on low and high ground have limited interaction. They can each just hide behind cover and be invincible. There's no way to attack the high ground without being on the high ground. There are ramps that let players traverse the map, but it takes so much time and commitment to get back up to the high ground once you've dropped to the low ground. With Stone Keep for example, high ground is an advantage, but you can still attack high ground enemies without having to be on the highest part of the map.