Why is used a cement unit pump when they make leak off test?

Using a cement unit give you the advantage of recording accurate  pumping pressure and the volume and give you the required   pressure and you can get charts for that in hard copy and some cement units provide soft copy also. You also can pumping with any rate even small rate equal to .25 Bbl or less.

Another secondary reason it´s becasuse cement pumps are more reliable than regular pumps when subject to high pressures. Thats particularly importat for older rigs. The pressures may also be measured by LWD tools if your cement gauges aren´t reliable, so there´s plenty of room for redundancy.

Yes it's true but i didn't ask what mean leak off test. I think that in a cement line we have hight pressure and a low volume and that can be easy monitoring. What you think about that?

the leak off test establish a practical  value of the input into the  fracture pressure and indicate the limit of the amount of pressure that can be applied tothe well bore over the next section of the drilled ol