Altcoins are alternative cryptocurrencies that were launched after the success of Bitcoin. They are often seen as better substitutes for Bitcoin, and their value is closely connected to Bitcoin's value for several reasons. Firstly, Bitcoin is the first and largest cryptocurrency by market cap, and it often sets the trend for the rest of the market. Secondly, historically, most crypto trading pairs have been denominated in Bitcoin, making it the standard unit of account for the cryptocurrency market. Thirdly, Bitcoin is often used as a market sentiment indicator, and its price movements can affect the overall market mood. Finally, many crypto traders use Bitcoin as a way to diversify their portfolio and hedge against market volatility.