For asset tracking (Aruba Tags) you will need 'observers' that can hear your tag. Your tag is just transmitting, just like Aruba beacons. Both are not connected to the network.
In the case of beacons for wayfinding or proximity, the mobile phone with App is the observer. It hears the beacons and from that information can determine it's own location.
With tags, the observers are BLE enabled access-points, which can be both Instant or controller APs from the 3xx series with built-in BLE radio or 2xx series with a USB port and USB beacon that acts like BLE radio.
Because the AP is connected to the network, it can report the received signals from the tags and the Meridian cloud can correlate that to a location of the tag if it hears the tag on multiple APs.
So the reason you need APs is that you need devices that can receive BLE signals from the tags.