What are the benefits of applying for permanent residency after legally residing in Spain for 5 years?

Permanent residency allows the individual to legally reside in Spain indefinitely (you will have to renovate your residency card every 10 years). 

After obtaining permanent residency you won’t need to provide any more proof of sufficient financial means or health cover, and you will enjoy full access to the Spanish healthcare system on the same basis as any other Spanish citizen. In addition, you will be able to leave the country for 5 consecutive years before losing your permanent resident status, in contrast to the 6 months granted by temporary residency.