Many people put a large portion, if not all, of their money within their homes. This is often the result of their suspicion/disdain towards investing and banks. Amongst the rest of this group, many, including myself, keep their banknotes and coins within grasp as a hobby.
One time I overheard someone saying that a coin collector would never be poor (or go broke). This may be one of the advantages of coin collecting — if a coin collector runs into money trouble, one can use one's collection of local currency directly to pay off some or all of the money owed. (The downside is that one's collection would likely bail out its owner only once.)
To build a collection of the local currency, one only needs to collect the coins and banknotes of high quality. From there a collector often begin taking close looks at one's change to see if any coins and banknotes contain features such as misprints, special designs, and meaningful special serial numbers. This habit often manifest itself when a coin collector travels abroad.
When travelling abroad, studying the local coins and banknotes can help one learn more about the country they are travelling to. This is because banknotes and, to a lesser extent, coins often provide glimpses on the issuing country's history and culture — banknotes and coins often feature local landmarks, activies and artwork unique in the country, landmark discoveries discovered by the country's citizens. One may even learn the basics of the languages of the issuing countries and, by looking at the people featured on coins/banknotes, local politics.
Some people collect coins and banknotes as a way to invest. In the short term, this way of investing yields very little return, as the value of one's collection often remains stagnant. Coins and banknotes currently in circulation do not go up in value, unless the notes are misprints or have special serial numbers. The value of coins and banknotes often do not grow until they are no longer minted/printed and, later, removed from circulation. So if coin collectors hope to increase the value of one type of coin/banknote, they need to start hoarding that type of coin/banknote and to demand production of such coin/banknote be halted.
Sometimes coins/banknotes are issued in commemoration of certain landmark events or anniversaries. If someone wishes to purchase them for the sole purpose of making money, they need to sell them while the buzz surrounding the coins'/banknotes' release is high, as such items cost multiple of their face value and demand for these items often plummet quickly after the initial period. In most cases, the issuers of the coins/banknotes are the ones making a killing from the release of the coins/banknotes.
By collecting coins, the collector has something to fall back on when one has money issues, but one should not count of collecting coins to strike it rich.