"For two very good reasons:
The first reason is a practical one.
Suppose you have 5 sacks of grain to share between 8 people, so each would receive 5/8 of a sack of grain in terms of present-day fractions. How are you going to do it simply, without using a calculator? You could try pouring the 5 sacks of grain into 8 heaps and, by carefully comparing them, perhaps by weighing them against each other, balance them so they are all the same! But is there a better way? We will see that using unit fractions makes this easier.
The second reason is that it is much easier to compare fractions using Egyptian fractions than it is by using our present-day notation for fractions! For instance:
Which is bigger: 5/8 or 4/7? You cannot use a calculator.
On this page we see how both of these work in Egyptian fractions."

Ancient Egyptian Fractions - Strange, but better?