Can some one tell me why is one of these diagraming software

Hi Suka,
There are many more 'modelling' or 'idea' software packages than you list.
I have tried many of them.

I have found that different packages have different strengths and weaknesses in the different types of presentation they offer, and in the different ways they allow presentation and interpretation of the data (and ease of use).

Add to those facts that we each tend think in different ways and you will realise that for any task, the ideal solution would be to try all packages and then settle for the best one for that problem and similar future problems.

Since there are so many packages you will have realised by now that this is an impossible way to decide which package to use for which problem.

So, I tend to try several packages (without any specific problem in mind) to identify their strengths of data entry and presentation for analysis or understanding etc.
Remembering this info. allows me to choose which package to use on different types of newly arising problems.

To show relationships I love 'The Brain', which is very visual.
I have other favourites for flowcharts and decision making, (but realise that others will choose different packages).

I've used Visio for software engineering diagrams and whatnot. It's pretty nice. It has a ton of templates and object sets. If you draw lines connecting two objects and then move the object, it will redraw the lines with the least number of intersections possible.