To me personally, there are reasons that a solo album makes a heck of a lot of sense, but on the other hand there are times when it makes no sense at all. The times that it does make sense is when an artist has music inside of them, but cannot communicate those ideas within their current band or group. Albert Hammond Junior of the Strokes is a key example of this. It is a well-known fact that Julian Casablancas, the lead singer of the group has a very difficult time allowing other ideas to come across when they are making the new album. Albert needed to go outside the group to express his music and once he did so, everyone discovered that he was a pretty good artist on his own.
Another reason that I find valid for an artist to go outside of the band or group is that they wish to get more experimental than any album ever made by that band. I think of Julian Casablancas and the Voidz or Thom Yorke of Radiohead in this instance. Artists such as these wish to expand their sound, experiment with their sound, but doing so in the natural framework of their original band might cause issues. Some of the possible consequences of doing so in the original band would be turning off fans, underselling an album, alienating the other band members. Sometimes going outside the group would be the only way for the music to see the light of day as other band members may not want to go that experimental. Being experimental is interesting and fun, but people do start to get scared when it becomes risky and affects potential earnings growth.
Another time that a solo album makes sense is that a particular artist in the group is going to make a complete break from the group anyway. This has happened over the course of time throughout music history. You think of an artist like Rod Stewart or more recently Noel Gallagher, who really needed to be on his own due to his enormous ego breaking free. The fact that this happens is sometimes sad, but explains the complexities of group dynamics and smacks of the inevitable.
Now, there are other times when a solo album seems pretty pointless. One of the examples in the recent past that I remember is the one done by Brandon Flowers of the Killers. The album was basically made up of songs that had been rejected by the band. If you play the album, the average listener probably could not tell the difference between that and a regular Killers album. I believe what happens is a driving force in a band from the songwriting standpoint gets really anxious to create and produce new music, but other band members simply do not want to record at that time. Instead of being patient and waiting for the band members to come around, they simply go ahead and call it a solo album. One of the issues with this is the fact that the other bandmembers do finally come around and are ready to record, but that songwriter is not ready to record because he is out touring his solo album. I remember Julian Casablanca's during the recording of the album Angles recorded his vocals on his own and emailed them to the group. This really is not a very effective way to make an album.
The solo album simply put is a complex creature. You never know the dynamics or the emotions behind it. So album could be quite harmless, but it also could cause damage to a band that is beyond repair. That is before the album is even made, toured with, and received by the public. I guess for me with some of my favorite bands when a solo album is on the horizon; I get a bit angry and jealous because I simply do not want to wait around even longer for the band to come out with new material. Let me close by saying that I think that in rock 'n roll the solo album becomes a much more complicated venture because of the prevalence of bands or groups when compared with other genres of music like country and rap where the entire substance of that kind of music is solo related.