These coolant expansion tanks are useful on N54 powered vehicles that have either relocated turbo inlets or have been converted to a single turbo as it allows you to relocate the expansion tank to the other side of the engine bay.
The coolant expansion tank in your vehicle gives additional fluid a place to go after expansion and contraction from temperature change. Because it is subjected to such drastic changes on a regular basis, it can often crack or break. You want to keep all the coolant in your vehicle in the cooling system. If your tank has experienced damage such as cracking, nipples breaking off, or mounting point damage, you will need to replace it.