The Values area controls both aggregation and display. Summarize Values By answers a question such as total revenue, average sale or count of nonblank entries. Sum ignores blank cells. Average includes numeric zero, but it doesn't treat a blank as zero. Count counts nonblank records, including text.
Show Values As transforms an aggregate after the groups have been calculated. A percentage of Grand Total uses the whole report as its denominator. A percentage of Row Total or Column Total uses the corresponding row or column instead. These are different questions even when some percentages happen to match.
Use Value Field Settings for both choices and the field's number format. Running Total In, Difference From and Rank also need a base field. A rank tie shares the same rank and leaves a gap before the next rank.