If your device supports Dual SIM (the ability to use two SIMs at once), you can absolutely choose to leave your primary cellular plan "on" while using your aloSIM plan.
As long as you make sure data roaming is "off" for your primary cellular plan, you won't rack up any roaming charges.
BENEFIT OF LEAVING IT ON
The advantage of keeping your primary cellular plan on while you're also using an eSIM data plan is that you're fully reachable on your regular phone number.
If there's an emergency back home while you're traveling, you'll continue to receive calls and texts. There's no need to worry you're missing anything because people don't know how to contact you in another way.
You won't be charged roaming fees (as long as you keep data roaming "off" for your primary cellular plan) because you'll be using your aloSIM plan to maintain a great data connection.
COST OF LEAVING IT ON
However, if you choose to answer/make phone calls on your primary cellular plan while you're outside of your coverage area, your carrier may charge you per minute for those calls. (There won't be a charge for calls you choose not to answer.)
And if you choose to send/receive texts on your primary cellular plan while you're outside of your coverage area, your carrier may charge you a flat rate for each text sent or received.
If you plan to keep your primary cellular line "on" while you're traveling, it's a good idea to check with your carrier to see how much they plan on charging you for calls and/or texts. Then you'll have a clear understanding of what you'll pay to talk/text while you're away.
HOW DO I DECIDE?
Some aloSIM customers choose to keep their primary cellular plan turned "on" while they're traveling because they want to stay connected via the phone number everyone knows, and others decide to switch "off" their primary line because they don't mind being reachable in a different (free) way, like a temporary phone number.