Is it always necessary to use a CSS grid when you're creating a web app?

No it is NOT but RECOMMENDED. Why?

1. Chances are the web app you'll be doing should cater different screen size, GRID can handle that without or little issue rather solving task for every device / platform it should cater.

2. Using GRID or even responsive GRID promote good UI/Interface, no need to do or tweak a lot of things to make it presentable.

3.  Lastly, somebody or someone already solve the problem related to browser issues in CSS/JS why need to worry about it and just use the one tried and tested solution like GRID... in short, no need to re-invent the wheel unless it's something truly innovative from existing one.

its depend to your design, your framework but usually i prefer to use CSS grid system.

Connect more than 30 services into your app with simple integration. We don't support a website that you use, yet? Just insert the site link and watch the magic

it's not necessary, but it make much easier using grid systems, you will lose time if you make by your own.

No but when you use it you will save your time to Design other one

no that is not necessary but that using to give your site and controls an good look :) 

Not really! But following the tried industry practices is always beneficial specially on professional level.

Agreed with @Clodelio.

It is recommended because grid system can help you to organise modules and sections of code which is not just useful for us as designers or developers but infact they’re good for the users too with respect to user experience.

Using a grid system in your designs is one way to achieve a level of consistency that would be otherwise extremely difficult to master and to portray in your designs.

 

That's why CSS GRID is recommended. And yes - Its always best not to Re-Invent the wheel :-)

There's no specific answer on your question (nor yes or no). its depends on the application that you're working on, and the "Client's" requirements. Feel free to ask for further information, best of luck

No, not always, but its depends on your need, like BootStrap responsive Grid System can bring website layout design to the next level and now a day's most popular framework to build fast and responsive. Another very unique and intuitive options  Google Material Design

All of these based on end users requirements and how beautiful way we can represent by UX/UI

 

its a good practice to use CSS grid framework to make a design based on some grid, however it is not always necessary.