Think of it this way
Part 1
1* Heroes give the best +% Skill roll per XP. Since your skill rolls are limited by how much XP a hero takes to level, 1* Heroes are the best ( there are special circumstances).
5* Heroes have the best special skills, you they are the most important to increase ( Note 1 )
Part 2
You have a week of vacation to finish a roof repair. You can leave work 1 hour early for 56 days ( roll one 1* hero ) or take the whole day for 1 week (batch of ten 1* heroes ).
You cannot work on the roof repair when the weather is bad ( fail your skill rating increase ). So leaving work early for 56 days gives you a better chance being able to work ( getting some increase ) Edit: because it is very unlikely to have a 56 day storm. But if you take the whole week off, and a 10 day storm shows up, you get zero work done ( no increase except ascension increases ) where it would have only ruined 10 hours if you had used the leave early option.
But the odds are better for 7 days in a row of good weather than 56 days in a row, so each individual day ( skill roll ) has a better chance of being good ( increasing the special skill ) with taking the whole day.
Basically the whole day decision ( batches of ten ) lumps a lot of your chances together for good or bad. Where leaving early ( batches of one ) spreads out your chances so you only miss the normal amount of days due to bad weather.
But even if the weather is really good all week or all 56 days, you may find unexpected damage and not have enough time to finish the repair before you run out of time ( you roll bad in batches of one or batches of ten and still get 3/8 3* 3.50 hero ).
Note 1)