Coin market-cap is a flawed measuring instrument. Still, it has descriptive value as a shared data point for market participants. Human beings, influenced by emotions and group psychology. There is a common cognitive bias called the "unit bias" where the notional unit of a good service irrationally influences human decision making.
What sounds better? Seven hundred and Sixty-Five Million (total market cap of bts as displayed on https://coinmarketcap.com/currencies/bitshares @ 05.10.18)
or....One Billion?
Simple and memorable. But only because of the unit of account (USD). Combined with another cognitive bias - the anchoring effect - some valuations may gain their own inertia and justify their relative value if a sufficient part of the market participants associate a specific number as a price equilibrium. This helps explain why some coins hold their relative value to others.
Full disclosure, I do hold bitshares. I am a long term hodler because I believe the network value of a working Decentralized Exchange is well in excess of one billion usd. In this author's opinion, bitshares is a useful and undervalued asset.