Scientists use models to show how atoms bond, but every model leaves something out or shows something that is not real. You need to know what each one is good for and where it falls short.
A dot and cross diagram shows only the outer shell electrons, with dots for the electrons from one atom and crosses for the electrons from the other. It is good for showing which atom each electron came from, and whether electrons are transferred or shared. But it is flat, so it does not show the shape of a molecule. It does not show the relative sizes of the atoms or ions, and it does not show the lattice of an ionic compound. All electrons are identical, but the dots and crosses make them look different.
A ball and stick model shows atoms as balls and bonds as sticks. It shows the three-dimensional shape of a molecule and how the atoms are arranged in space. Its limitations are that the sticks make bonds look like physical objects, when a covalent bond is really a shared pair of electrons and an ionic bond is an electrostatic attraction. The model does not show electrons, the balls are not in the right relative sizes, and the gaps between the atoms are far too large. In an ionic compound the sticks suggest bonds between pairs of ions, but each ion really attracts all its neighbours in the lattice.
A two-dimensional drawing, such as methane with lines for bonds, is quick and clear but it is flat, so it cannot show the true shape, which is three-dimensional. A three-dimensional model shows the shape better, but it is harder to draw or share on paper and atoms at the back can be hidden. No model is perfect, so you choose the one that shows what you need.