Many altcoins are created with different purposes. I am not sure why only Ripple is being compared? In reality, altcoins shouldn't be compared with Bitcoin at all. There are multiple reasons alongside centralization. Also I am not against altcoins. There are many benefits altcoins provide.
Bitcoin is more like pioneer in this market and enjoys the first mover advantage. But altcoins are also needed, especially coins like Monero. In this world of surveillance, privacy coins are really beneficial.
Yes, and one of the greatest benefits that altcions offers is being an alternative to Bitcoin for individuals, cooperate organizations and companies uses.
Again, altcions offers competition on the crypto market, once there is a market, it comprises of different coins/tokens that should be traded, or paired to each other to create opportunity at intervals.
In facts, Bitcoin will not strive on the market alone, there should be other coins/tokens to create trading, investing activities, similar to fiats market pairings with each other at the FX (foreign exchange) market.