Sometimes stamp collecting can be financially rewarding, but this generally not the case for the majority of collectors.
Many hobbies do not involve making money, but they are still very rewarding. So is stamp collecting. The rewards come in the form of knowledge, relaxation and friendships gained from pursuing your hobby.
If you have an expectation that collecting stamps ought to reward you financially, you must not expect to achieve this aim in the short term. Beginner’s prospects for collecting stamps that will grow in value are not necessarily good, because beginners lack sufficient knowledge. Without investing in years of effort building this knowledge, it's not easy to recognise opportunities for acquiring worthwhile stamps. Naturally, there are no certainties in the process and even experts can’t be sure their stamp purchases will prove lucrative.
Likewise, the beginner should be prepared to buy stamps and accessories without an expectation they'll rise in value. It's more important to enjoy your stamp collecting.
The overwhelming majority of stamps you can collect are not valuable and can be purchased for relatively little, because they're produced in large quantities serving a postal requirement, as well as meeting demand from collectors. The production quantities of most stamps are invariably so great, they rarely appreciate in value after their withdrawal at Post Offices. It is rather pointless buying current stamps with a financial motive in mind. You should buy new stamps for their appeal and the enjoyment you will get adding them to your collection.
The “secret” to making money out of stamp collecting is to buy scarce stamps and other philatelic items (such as postal history) at affordable prices and retain them in the hope that interest in these collecting fields becomes more widespread. In this event, prices will probably rise and your collection is more likely to be of greater value than what it cost you to put together.
Of course, it's impossible to know precisely which “underrated” areas of stamp collecting will become popular areas in the future. Experienced collectors know the joy of the hobby lies in building collections around themes that interest them – the prospect of financial gain is secondary.