The initial assignments are used to assign initial values of a species, e.g. tumour size, inital concentrations of the species in your model. But also to calculate a model parameter at the begining of the simulation, e.g. baseline tumour volume given inital tumour cell number.
A repeated assignment could be used to update a parameter one would create to express an outcome of your model which is not a specie. Examples are: the volume of the tumour (calculated based on growing/shrinking tumour cell number), total sum of certain cells in your model, i.e. all immune cells, at any time point.
For detailed overview, see assignments in SimBiology
