The Lockdown mechanic is a new feature in Persona 5 where people can put in their persona. When they do so, their persona is “locked” inside a cage. While it stays in the cage, it’s exp will go up even without you using it. This is useful for those persona you need to use as fusion materials especially when fulfilling Justine and Caroline’s requirements.
However, when using Lockdown, this only shows effects after the persona has increased in a level. That being said, it may sometimes be an obstruction. Because it’s put in a cage, you can’t use it in combat nor can you replace it when you have full stock. Thus, it might appear more as a place holder and a waste of space. But if you wish to use the Lockdown system, try using it when you’re just running around from place to place. As you do so, the days pass by and the persona may gain more EXP.
However, do be careful when using this mechanic. If you want to quickly swap persona then, best don’t use this feature. Use the feature if you have an extremely low level persona. Although personally, it’s better to just run and train them in Mementos while relying on your teammates to carry.