• as the other commenter mentioned, it's much less likely that you'll repeat the same error, because you'd have to do it in a totally different way;
• if the numbers are more than two digits, it's much less work to do casting out nines than to redo the calculation. Multiplying 38020562 by 95942, using the standard algorithm, you have to do 40 digit multiplications, 32 carries, and then sum five six-digit partial products (about another 25 additions) to get your final sum of 14 or so digits; that's a total of 97 operations. Checking it by casting out nines involves only about 26 operations, which can be simplified further by casting out some nines ahead of time — 95942, for example, is obviously 2 mod 9 because the other digits are 9, 9, and 5+4 = 9.
(Side note: is it worthwhile to use Karatsuba multiplication to multiply 38020562 by 95942 by hand?)
Check calculations like this are much easier to do mentally, and provide a rough measure of confidence in the rightness of the result.