The User ( you ) do realize that Time Machine Backup once enabled is making Snap Shots at the time it is scheduled to perform a Backup regardless whether the the External Drive is attached or not.
Given enough missed executions to transfer the Snap Shot to the external Drive - the Internal Drive is accumulating Snap Shot after Snap Shot and will continue to do so until the External Drive is actually attached.
This will increase the risk of the Internal Drive getting so full that a Warning may appear from the Operating System of " Running Out of Space "
If left unattended to, this could lead various unintended things to occur , including Decreased performance due to lack of Empty Space, Increased CPU and Memory usage as the computer has to SWAP out from RAM and Back into RAM plus increased Battery Usage to perform all these addition tasks.