Changing your gun up can make your gun feel and handle better (not to mention look good) but you might possibly add weight and require someone to know what they are doing to install these parts.
Detachable grips/vertical grips/vert grips/tactical grips/etc are designed to attach to a gun and give your offhand a place to hold your rifle or SMG from. This gives you more positive control over certain situations or can help the shooter control recoil better (not an issue in airsoft so much). Certain grips also have added features sometimes such as storage compartments, pressure switch slots, retractable bipods, folding bodies, and so on.
Some grips are more basic and are just literally only grips. There are stubby grips as well which offer a minimalist approach to giving you a vertical grip without excess length which can add weight or can be more clumsy in some users eyes. Angled foregrips are designed to give you a something to grab on that is low profile but more properly lined up with the bones in your wrist giving you better comfort and control.
The front part of your long gun (excluding shotguns) is called the handguard. This is where you would hold your rifle with your off hand and its purpose is to keep you from touching the barrel as it functions as sort of a heatshield. Heat is not a problem in airsoft guns fortunately so this is just a part of the gun you hold. Handguards can be swapped out for other styles and shapes on the more popular guns. Many people change handguards over to forends with rail systems so they have more options for accessories. Take for example the classic M4 with a round handguard. With a gun like that you have numerous aftermarket options for handguards. You can change to a different looking one, one that has rails, and so on.
A popular thing people change over to are not for standard picatinny rail systems so much as Keymod or MLOK styles which can accommodate modular accessories and still keep the gun very streamlined. This is all user preference as the goal at the end of the day is to personalize your airsoft gun the way you want for the best function and comfort.
Do keep in mind that though there is a large aftermarket for Keymod or MLOK specific accessories, these are somewhat more proprietary than your standard Picatinny rail and add cost. One example is if you want to take a grip meant for picatinny rails and want to mount them to a Keymod handguard. You will have to add a picatinny to keymod rail section to make this happen. Or you could get a Keymod mounted grip and attach it directly to the Keymod compatible handguard. So you can see how this would add to your cost a little bit, though the latter solution would offer a more streamlined package. Again this isn't going to make or break your gun but a minor detail people often come across when customizing their gun to suit their needs.