This has been a question most newbie or professionals don’t understand, but to be honest with you, it’s not about needing an extra monitor but wanting it and this is the reason:
TO GET EXTRA INFORMATION YOU WANT TO GLANCE AT WHILE WORKING ON A PROGRAM: This could be system statistics or browsing something on the main monitor and this could be very helpful because instead of minimizing, searching through all the opened windows to get that information on the PC you currently using while working on a program, You just glanced at that information on the main monitor or second PC.
NOTE: Minimizing on a PC with a low RAM or a filled RAM takes time to run due to unclear RAM cache and many others.
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CAN I AFFORD A SECOND PC OR A MONITOR?
Honestly, I would advise a second monitor or a monitor instead of a PC and the reason for this is because the main aim for getting a monitor is to get an extra information on a concept while coding or working on a program and it would be best to use a large screen for that, and this is were a good monitor comes in.
NOTE: You don’t need to buy the most expensive or flashy monitor because the aim here is not to showcase your working tools but the aim here is to save time and improve on efficiency and yeah you can actually ship an old monitor too.