I can't speak for everyone but there is generally less hassle involved with just closing the lid and reopening it, instead of shutting it down completely. You don't have to wait for it to shut down or start up; you're right where you left off when you closed the lid. If you have any downloads or processes ongoing, they can continue right where you left off for the most part, instead of having to restart completely. Imagine being in the middle of converting a video which takes 2 hours, but you have to go somewhere else with your laptop; it can make a huge difference if the conversion continues where it left off afterwards or needs to restart completely from scratch, making the previous waiting time obsolete.
There are also other features to just putting the laptop in standby. You can unlock your MBP via Touch ID whereas after a restart, it always requires your password first. And you can use Apple's power nap feature that lets it do certain tasks (checking for new messages, E-Mails, notifications, etc.) even when in standby.
Apple's intention for the standby mode seems to be to make it more like what it is on iOS devices. I.e. you don't turn your iPhone or iPad off completely whenever you put them into your pocket/backpack, you just put them into standby. Similarly Apple wants to make the usage of the MBPs more seamless by having you be able to close it whenever you want, open it up again a few hours later and continue where you left off. I mean starting the MBP doesn't take anywhere as long as it did a few years ago, but if you're doing it like 20-30 times a day whenever you don't use the device for a bit, these times can absolutely add up.
