LONDON, May 18 (Reuters) - Bitcoin's smaller rivals are eroding its share of the $2 trillion digital currency market. Of the dozens of other cryptocurrencies, most have no use other than financial transactions - but few of the investors driving their rise are troubled.

Among the major "cottage coins" - the collective name for all cryptocurrencies except Bitcoin - the likes of ethereum aspire to be the backbone of the future financial system. Cryptocurrencies such as dogcoin have no such ambition and are rarely used in payments or business.

For retail investors who throw their money at these currencies, the backstory of these currencies - and the volatility that can make them suffer serious losses - is usually irrelevant.

What buyers see is the opportunity to make a quick profit, or at least an enjoyable experience.

Bitcoin's share of the cryptocurrency market has fallen from 70 percent to about 45 percent this year as investors flock to cottage coins, according to U.S. research firm Coin Metrics. Data provider CryptoCompare said bitcoin's share of volume on major exchange Binance halved to 23 percent.