When we used the sample space {Rain, No Rain} in Example 1 (Refer to NCERT page no. 160), we focused only on whether it will rain or not. If we want to include different amounts of rainfall like drizzle, light rain or heavy rain, we need to expand the sample space to {No Rain, Drizzle, Light Rain, Heavy Rain} so that it better matches the level of detail required for the question. It is important to ensure the sample space is detailed enough to suit the specific problem being studied.                                                                                                                                 [Page No. 167]

The sample space we choose should match the question we are trying to answer. If we only care whether it rains or not, then {Rain, No Rain} is enough; it lists every outcome we care about, with none repeated.
But if the question is about how much it rains, this two-outcome space is too coarse, because the single label 'Rain' lumps together a light drizzle and a heavy downpour, which are really very different events.

To answer the more detailed question we expand the space to {No Rain, Drizzle, Light Rain, Heavy Rain}, so each possibility we want to tell apart appears as its own separate outcome. The general rule is that a good sample space must (i) include every possible outcome, (ii) list no outcome more than once, and (iii) be just detailed enough for the specific problem, fine enough to capture what we want to study, but no finer than necessary. So the 'right' sample space depends on the level of detail the question demands.