StringExpression: Give all pairs of identical characters in the string

While going through StringExpression (https://reference.wolfram.com/language/ref/StringExpression.html), unable to figure out the following:

Give all pairs of identical characters in the string:

Input:

StringCases("abbcbccaabbabccaa", x_ ~~ x_)

Output:

{bb,cc,aa,bb,cc,aa)

Why is x_ ~~ x_ used?