To translate a vector by 10 unit in the X direction, why do we have to use a matrix?
We can just add 10 to the mat, and we got the same result too.
Yes, you can add a vector in the case of translation. The reason to use a matrix boils down to having a uniform way to handle different combined transformations.
For example, rotation is usually done using a matrix (check @MickLH comment for other ways to deal with rotations), so in order to deal with multiple transformations (rotation/translation/scaling/projection...etc) in a uniform way, you need to encode them in a matrix.
Well, more technically speaking; a transformation is mapping a point/vector to another point/vector.
where p` is the transformed point and T(p) is the transformation function.
Given that we don't use a matrix we need to do this to combine multiple transformations:
Not only can a matrix combine multiple types of transformations into a single matrix (e.g. affine, linear, projective).
Using a matrix gives us the opportunity to combine chains of transformations and then batch multiply them. This saves us a ton of cycles usually by the GPU (thanks to @ChristianRau for pointing it out).
It's also good to point out that GPUs and even some CPUs are optimized for vector operations; CPUs using SIMD and GPUs being data driven parallel processors by design, so using matrices fits perfectly with hardware acceleration (actually, GPUs were designed to fit matrix/vector operations).
If all you are ever going to do is move along a single axis and never apply any other transformation then what you are suggesting is fine.
The real power of using a matrix is that you can easily concatenate a series of complex operations together, and apply the same series of operations to multiple objects.
Most cases aren't that simple and if you rotate you object first, and want to transform along its local axes instead of the world axes you'll find you can't simply add 10 to one of the numbers and have it work out correctly.
the reason to use a 4x4 matrix is so that the operation is a linear transformation. this is an example of homogeneous coordinates. The same thing is done in the 2d case (using a 3x3 matrix). The reason for using homogeneous coordinates is so that all 3 geometric tansformations can be done using one operation; otherwise one would need to do a 3x3 matrix multiply and a 3x3 matrix addition (for the translation). this link from cegprakash is useful.
To succinctly answer the "why" question, it's because a 4x4 matrix can describe rotation, translation, and scaling operations all at once. Being able to describe any of these in a consistent manner simplifies a lot of things.
Different kinds of transformations can be more simply represented with a different mathematical operations. As you note, translation can be done just by adding. Uniform scaling by multiplying by a scalar. But an appropriately crafted 4x4 matrix can do anything. So using 4x4's consistently makes code and interfaces much simpler. You pay some complexity in understanding these 4x4's, but then lots of things get easier and faster because of it.
See this video to understand the concepts of model, view and projection.
4x4 matrices are not just used for translating a 3D object. But also for various other purposes.
See this to understand how the vertices in the world are represented as 4D Matrices and how they are transformed.
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