Assertions allow you to put checks in to the code which are checked when the program is run from within the interpretor. So for example it could be used to check values being passed into a block.
//---------------------------------------
// File: ASSERT.e
//---------------------------------------
//---------------------------------------
// Block: DivideTest
//---------------------------------------
// Variables:
// Value1
// Value2
//---------------------------------------
block DivideTest Value1 Value2
ASSERT Value2 == 0 "Division by 0"
Answer = Value1 / Value2
outl Answer
_block
//---------------------------------------
// Block: go
//---------------------------------------
block Go
DivideTest 20 5
DivideTest 20 4
DivideTest 20 0
DivideTest 20 2
_block