Your muscles provide a stabilising role to your spine and surrounding joints. Weak core muscles mean your spine has to endure more load and stress, this is something you NEED to address in order to have a healthy spine. This video demonstates the bird dog exercise which helps improve your extensor endurance for your low back.
The extensor muscles are primarily the group of muscles in your back which hold your spine up. Keeping these erector spinae muscles healthy and strong is important for the ability to maintain a healthy posture. These muscles are activated when we lean back, and thus the stronger they are, the less likely you are to damage your spine.
3 sets of 10 repetitions are recommended. The amount should be decided by how you tolerate it. Eventually you should be able to complete 3 lots of 3 sets per day but it is ok to start by doing 3 sets once per 2-3 days until strength is built up. Flexors, lateral flexors and extensors should be strengthened equally.