Closing apps on my iPad, why is it bad/

Some understanding of how memory management works within iOS/iPadOS might help. To be clear, this is not intended to be an in-depth explanation, but will hopefully capture the essentials in basic terms...:

Your iPad/iPhone will always attempt to use most of its available RAM - unused RAM in this low power/high performance system architecture is essentially wasted resource. 

iOS Apps are generally in one of four states - the first three are the most relevant.

• The App is “Active” - it is running running in the foreground. When you switch tasks, the App will continue to run in active state for some minutes before its resources are released and is placed into an Inactive state.
• The App is “Inactive” but remains loaded in [fast] RAM. In this state, the App can be instantly restored to an Active state - but is not consuming CPU or other resources whilst in the inactive state.
• The App is “Inactive” and unloaded. In this state, the App has been completely offloaded (releasing RAM for use by other processes) but its running state has been saved to [slow] flash memory. Returning to an App in this state will cause the App’s saved state to be reloaded from flash memory into Active RAM - without the need to re-initialise the App.
• The App has been closed. All running data has been expelled - there is no “saved” state; relaunch will reload and re-initialise the App from scratch.

Memory management is generally a juggling act - and for the most part, you should allow the OS to manage its memory space. Re-initialising an App is the slowest and most power hungry method of launching an App. Inactive Apps do consume some system resources, but by design, the required system resources to maintain this state are insignificant.

Force-closing an App releases all resources. Whilst the mechanism exists to do so (and in rare circumstances manual intervention to terminate an App may be necessary), a force close reduces system performance.

Yes, open apps will not suck battery life unless there is something wrong with your device. It is not necessary to force close an app unless it is not responding. The iPhone and iPad handle RAM very well, and if you force close apps, it actually takes more battery power to open the app from being closed than it does to just have the app load up after it has been suspended for a while.