I've seen most programmers, when they declare an array globally, they take some extra spaces. for example, if the question asks for a 10^7 sized array they declare like this :
const int N = 1e7+10
int a[N]
why 10 extra spaces?
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When you do some operations on arrays which involve complicated indices (such as DP recursions), you can and often will access some index that is beyond $$$n$$$, such as $$$n+1$$$ or $$$n+2$$$. Usually it's just quicker to just add an arbitrary +5 or +10 than to calculate the exact number of extra spaces you need in the array.
(I personally don't do this +5/+10 thing, because I feel that this makes me lazy in thinking, but oh well, lots of people are doing it and they're just fine.)
it just feels good
It gives you +10 extra CF rating
omg sir, so if I do
will I become legendairy glandmaster and beat tourists on Codeforce??!1!1
You can get a maximum of 10 points for each problem you submit, but be cautious because a +10 in the array can also decrease your rating by 10 if you submited it wrong.
So make sure you choose them carefully