The adjustable charging levels are only useful if the circuit you are tapping into must support other items at the same time. Ideally, it is a dedicated circuit, then, maximum would always be the norm.
FWIW, the EVSE sends out a signal announcing how much current it is designed for, and the car then will decide how much, up to that maximum or its internal capacity, that it wishes to use. So, say you had an 80A capable EVSE, plugging in an i3 to it would not overload either the EVSE or the i3 as the i3 would only draw up to it's maximum capacity. Sort of like plugging in a 100W lamp into a plug capable of 1800W (typical 15A circuit) except the receptacle is smarter than a typical wall outlet.
BMW chose 12A for the USA supplied level 1 EVSE to support the most common receptacle in US homes - a 15A device. Code says you can't draw more than 80% of the circuit capacity constantly, so that means 12A on that 15A circuit. Should you have a 20A/120vac circuit, theoretically, you could get 33% more, or 16A, but then, you'd not be able to plug it into a 15A receptacle. Finding a 20A receptacle in a typical home is just not going to happen unless it was installed for a specific purpose. This would severely limit the usefulness of the EVSE. A 20A/120vac receptacle has one slot T-shaped to accept either a 20A plug or a 15A one. A 20A device must come with the plug in that with one blade rotated 90-degrees so it can only be plugged into a 20A receptacle.