Why we use ID

I like to keep the functionality and presentation of my code separate, so I recommend using IDs for fragment identifiers and JavaScript hooks and classes for your styling. The benefits of this approach are that classes are more versatile because they can be reused, this makes your code more modular. Using IDs and classes for separate purposes avoids any potential specificity issues and makes your code easier to understand by another developer, which is important if you're working as part of a team.

Classes are used for global attributes, however, I recommend to all that you avoid using ID's, unless it is absolutely necessary. You can still achieve the same desired styling using a class without running into specificity issues later with ID's. You don't want to be putting ID's on everything to get your styles to come to life. Using classes will make your code much easier to work with, especially as your app grows. My two cents anyway, take it for what it's worth or don't.

In my experience, IDs are used when you want to specify something for styling purposes, if only one element should be styled that way. You can have several elements with the same class, but may want just one of them to be a little different. That is when I use IDs.

Let me know if you'd like more specifics.

I think you have to use ID's when creating html forms

This is the way I understand it and so far, it's been working well for me. Classes are, so to speak, css global attributes. That is, they can be used in any element and are very versatile. IDs are not. IDs are best used for manipulating specific elements via JS or CSS.