Here's the honest motivation before any notation: every piece of data a machine learning model touches gets turned into numbers, and those numbers get organized into vectors and matrices so the computer can do millions of calculations on them at once instead of one at a time. An image is a matrix of pixel brightness values. A sentence becomes a list of numbers (you'll see exactly how in Tier 4). A dataset of houses with their size, location, and price is a matrix where each row is one house.
1-A 2-A 3-A 4-A | 5 one 6 columns 7 = (3,2): [[1,2],[3,4],[5,6]] 8 = Vectorised math on whole datasets is fast (GPU/BLAS) 9 = Number; ordered list; grid of numbers | 10 = (28,28) matrix - flattened: 784-vector