Almost all audio sink devices will connect to the last connected source upon starting up. So far, so good – often that's exactly what I want, and it saves me from having to go into the source device's BT or sound menu to select the target device manually. This is great in cars, for example (unless you share a car).
But what many devices don't allow is "kicking out an existing source device" from a new connecting source device. This means that in order to use the sink, I'll need to chase down the currently connected source in my house and manually disconnect it (since sinks usually don't have any UI affordances to disconnect the current source without shutting down the sink).
Some newer Bluetooth sinks, including Apple's AirPods, do support that behavior, and it's been an absolute game changer for me. I really wish Bose would adopt it too; their "connect to two devices simultaneously" hack usually does way more harm than good, especially when some of these devices are shared in a household.