To maximize a window means to make it as large as possible. You will find yourself using a window's Maximize (square) button when a window opens too small. A window can open too small for two basic reasons. Either your Windows settings and/or a particular program's settings are set up to open each window small. Or you, or someone else, had each window opened small (custom size) and then closed them small.
Regardless of how a window became small, at some point you might need it open at its maximum size. And this is what the Maximize button is for. It allows a window to open (expand) to the maximum size of the desktop screen's display area. So if you are using a desktop screen size of 1024x768 the window will expand to a width of 1024 pixels (dots) wide and a height of 738 pixels (dots) high. The height shortage is due to the maximized window not covering the taskbar.
Fig 1.0 Viewing the contents of two non-maximized (custom sized) windows at once
In Fig 1.0 above I have two non-maximized (custom sized) windows open at once - The Microsoft Edge window is being used for my Internet research and the Notepad window is being used to take notes. As I continue my research I could carry on using the windows at the size they are now, but this would be impractical simply because I would need to scroll each window's contents downwards (and/or upwards) in order to view/write more contents. Small windows side-by-side are only good if you are reading their contents only. In my case though I am writing contents too. So the best solution is to maximize both windows, by clicking on their Maximize (square) button.
Fig 1.1 Click on a window's Maximize (square) button to make it full size (Maximized)
Maximizing both windows will enable me to see more contents and do less scrolling. The downside is I will have to flip between each window, by clicking on each window's taskbar icon (Fig 1.2 below), as both windows will not be in view at the same time.
Fig 1.2 Flip between windows by clicking on each window's taskbar icon
Fig 1.3 Viewing the contents of the maximized Internet Explorer window
When you maximize a window the Maximize (square) button changes into the Restore (double squares) button, and vice versa. Clicking on the Restore (double squares) button allows you to restore a window back to its original, last, custom size.
Fig 1.4 Click on a window's Maximize (square) button to make it full size (Maximized)
Fig 1.5 Click on a window's Restore button to restore the window back to its original (last), custom, size.
When you close a window, 99% of the time, Windows 10 remembers its position and size so that the next time it is opened it is exactly as you left it. I say 99% because some older versions of the Windows operating system, and some software programs, do not always save your window's position and size.
The disadvantage of maximizing all your windows, as explained above, is that you can only view one window's contents at a time; which means you can be switching between many windows all the time. Some people choose to re-size and/or move their windows whenever they need to so they can see more than one window's contents. However, their disadvantage is having to re-size and/or move each window into position more times than not. So the reality is; you will find yourself forever re-sizing, minimizing, maximizing and moving each window on a per needs basis.