Halliday & Matthiessen (2014: 144):
What these have in common is interpersonal deixis: that is, they locate the exchange within the semantic space that is opened up between speaker and listener. With primary tense, the dimension is that of time: primary tense construes time interpersonally, as defined by what is ‘present’ to you and me at the time of saying. With modality the dimension is that of assessment: modality construes a region of uncertainty where I can express, or ask you to express, an assessment of the validity of what is being said.