Allintitle is an operator that helps us query Google to find out how many titles (how many articles are indexed by Google and likely to appear on the SERP) contain a particular keyword. Since it tells us the number of articles about any given keyword, the Allintitle index indirectly tells us that if we choose to rank 1 for a keyword, how many other articles will we have to compete with. In other words, it reflects the competitiveness of the keyword in some way.