Think of it like a taxi. A low setting keeps you in a slow, expensive cab because you already paid the entry fee. A high setting kicks you out the moment a faster, cheaper cab pulls up next to you.
The benefits of roaming aggressiveness include: what is roaming aggressiveness in wifi
If your device is set to "low" roaming aggressiveness, it will cling to the original, weak access point for dear life, even when a better one is right next to it. This results in a "sticky client"—connected, but useless. Think of it like a taxi