Most of the casino sites that use their own games software will also use some other software brands as well as their own games. It is impossible to group these together as a collection of online casinos, as, because the games are their own, there is no relation between the different casinos software offerings. The only real reason to list them together here is that they do not go in any other software grouping.
Casinos that use licenced software have a bit of an advantage over these casinos as the players may already be familiar with the brand of games they expect to play. Sites that develop their own games need to do extra work to sell those games to their players.
There must be a compelling reason then, for a casino to choose to take on the effort and expense of developing their own software, instead of simply licencing some already existing and well known brand of games.
In the case of those casinos, such as Bovada and Slots.lv, that use a mixture of licenced games and some of their own ones, it must be due to their desire to either offer the best gaming experience that is possible, or to have a unique offering for their players to help bring about player loyalty.
If it is the second option, it certainly seems to work in the case of the above two casinos, as their brand loyalty is unsurpassed in teh gaming industry.