Lewis dot structures are a useful first step in drawing any molecule. They show us exactly how many valence electrons from each atom are making up the bonds or are sitting on an atom unused. Thus, it is generally easy to find the bonding and non-bonding electrons.
Here are some examples:
In any Lewis dot structure, the electrons placed in between the atoms are bonding electrons (they represent chemical bonds). At the same time, any electrons (or electron pairs rather) that are just sitting on atoms and not shared with another atom are non-bonding electrons. Non-bonding electrons in Lewis dot structures mean that those valence electrons are not involved in any chemical bonding but are simply occupying an orbital on a valence shell of an atom to which they belong.
How Lewis dot structures are useful?
Lewis dot structures show us the connectivity in molecules. On top of that, they also show which electrons are bonding and which ones are non-bonding. Lastly, they make it very easy to keep the electronic bookkeeping!
What are the problems with Lewis dot structures?
Drawing Lewis dot structures for a large molecule is tedious. Try drawing out all valence electrons as dots for a molecule containing 50 or 70 atoms! For instance, a common fatty acid stearic acid contains a total of 56 atoms. When you have to draw one of those, it might not be that big of a deal (it’ll be tedious though). Now imagine having to draw it several times in a chemical reaction? Well, you get the point.