When you reach 2K rep and have full edit capabilities and edit review privileges, you will begin to appreciate the minimum character requirement on the summaries. Sometimes I think it is too short, but making it more than 10 characters isn't practical.
It's kind of annoying and serves no immediately obvious purpose given how extensive and thorough the edit viewing system is in the first place-- it's very clear what edit was made.
You are partially correct, it is clear what edits were made, but often is it not clear why the edit was made. This is where the edit summary comes into play. When reviewing you only see the original and edited post side-by-side and you are looking at the changes. It is not uncommon to see an edit and say "why did they do that" and the summary is sometimes not more than "fixed answer", "improved formatting" or "clarified question" without any understanding where the fixes, improvements, or the clarifications are coming from.
The reason for the 10-character limit is to force editors into making an attempt at a clear edit summary. If you lower the threshold to 4 characters you open up to a lot of short words like "typo" or "fixed", and those are the types of words that the minimum is designed to prevent. There are certainly cases were 4 characters would be completely clear, but there are just as frequent cases where someone suggests an edit, puts down the minimum number of characters to fill the space and then wonders why the edit was rejected.
In addition to the edit review queue, the summaries also show up in the edit history of the post for the same reason. If someone happens to be looking at the edit summary, trying to get an idea as to why a particular edit was submitted, the summary is useful. If it is not clear, they could potentially rollback the edit.