By limiting to a low-risk segment of the population- military, older people, etc., an insurance and investment company can offer services such as car insurance at a lower rate than the larger companies- but a good insurance agency sorts for the lowest price for its customers. Many of us have found that to be the best route.

In the case of a company offering insurance, banking and savings, investment and brokerage services, such as USAA, certain of their services may be low cost with part of the risk covered by their brokerage and other branches.

USAA is a Fortune 500 financial services corporation; if one invests with them, one must trust the ability of their investment personnel just as one does with AIG, Goldman Sachs, etc.
In 2005, they ranked 191 on the Fortune 500 list, with assets of 46,500 million and stockholders equity 10,144 million; I don't have access to later figures.

The company seems to have a good, conservative image. Apart from that, if just one service is sought, such as car insurance, there probably are competitors who will do the job just as well.