Suppose you need the total of many different ranges of the same list: days 3 to 10, days 5 to 6, days 1 to 30, and so on. Adding each range from scratch is slow if there are lots of questions.

A prefix sum list stores running totals: prefix[i] is the sum of the first i items, with prefix[0] = 0. Then the sum of items from index a up to (not including) b is just prefix[b] - prefix[a], a single subtraction:

nums = [3, 1, 4, 1, 5, 9, 2, 6]
prefix = [0]
for n in nums:
    prefix.append(prefix[-1] + n)
print(prefix)

def range_sum(a, b):
    return prefix[b] - prefix[a]

print(range_sum(2, 5))
print(sum(nums[2:5]))

Building the prefix list is O(n) once; every question after that is O(1).