One benefit of using the above types of ring signatures over other
anonymizing techniques, such as CoinJoin or using coin mixing
services, is that they allow for “spontaneous” mixing. With CoinJoin
or coin mixers, it is similarly possible to hide the originator of a given
transaction, however these techniques in practice need some sort of
centralized group manager, such as a centralized CoinJoin server, where
transactions are combined by a trusted party. In the case that the
trusted party is compromised, the anonymity of the transaction is also
compromised.

Some coins such as Dashcoin, attempt to negate this by using
a larger number of trusted mixers (in this case masternodes) but this
number is still much smaller than the users of the coin.

In contrast, with a spontaneous ring signature, transactions can be created by the
owner of a given pubkey (this is the spontaneous, or “ad-hoc” property)
without relying on any trusted server, and thus providing for safer
anonymity.