Halliday & Matthiessen (2014: 585):
However, there are some respects in which they [i.e. desiderative clause nexuses] resemble nexuses of the verbal group.
(1) The projected element, a (typically perfective) non-finite, has – like the expansion types – given birth to what are now tenses of the verb, namely the two future forms will and be going to.
(2) The WH- probe is what does she want to do?, rather than simply what does she want?; compare what is she trying to do? not what is she trying?.
(3) The command forms – those with change of Subject – resemble some of the causative expansions; compare the following pairs, including the passives:
she wants him to do it she causes him/gets him to do it
he is wanted to do it he is caused/got to do it
she wants it (to be) done she causes it to be done/gets it done