There are two primary reasons to make a model variable a parameter.
- Output variables that are set as model parameters are added to the map when the model is run inside ModelBuilder.
- Variables that are set as model parameters show as tool parameters when the model is run as a geoprocessing tool, allowing you to specify different data or values for your model to process than what was specified when the model was created.
Hope this explains your query.
Model parameter so helpful for using creating model tool. that make a variable appear in the model tool dialog box, so you can specify the value. as it's say:
Any model variable can be made into a model parameter. When a variable is made into a model parameter, its value can be specified in the model tool dialog box
see that link : Creating model parameters—Help | ArcGIS Desktop