Once you've had a root canal, you will need a crown to support the tooth. Otherwise it is prone to crumbling apart. Other than that, no more issues with that tooth and it certainly doesn't affect any neighboring teeth. People need root canals because the tooth has died for some reason, or decay has gotten into the nerve chamber. A root canal simply removes the nerve(s) from the tooth, the tooth canals get cleaned out and smoothed, the canals are then filled, and a temperary filling is then placed. You then come back for another visit to have the tooth prepared for a crown, which fits over the tooth and basically holds the tooth together, which has now become weak. The worst thing about a root canal and crown is the hundreds to thousands of dollars you have to fork out!