Purchasing a safe with an inner steel liner provides many advantages to the safe owner. As most safe dealers know the weakest points of most any safes are usually the sides, back and top. Most safes today have only one exterior steel skin and fire insulation. The fire insulation can be in the form of gypsum board (or fireboard) or a poured concrete insulation barrier.
Why purchase a safe with a cement or concrete insulation barrier and an inner steel liner? The vast majority of safes sold on online marketplaces or through big box retailers are safes constructed having a 12 gauge (7/64") or 14 gauge (5/64") thick steel outer body. A burglar may use a portable wheel grinder that can quickly and easily cut through the outer steel skin if the steel is 12 gauge (7/64") or thinner (14,16 or even 18 gauge)...Then only the gypsum insulation board stands between a burglar and your most valued possessions.
Most fire insulation boards found within a safe are soft and brittle, and quite easy to break through with any common hand tool. When the safe has a concrete or cement insulation barrier it adds a significant amount of time to gain entry to the contents within the safe. The intruder would need at the very least a hammer and cement chisel to crack through the cement or concrete insulation barrier. This process takes time and is quite noisy and of course may attract attention. By having an additional inner steel liner you introduced a third and formidable barrier for the burglar to defeat to get to your most valued possessions. This third barrier also adds to the time element and creates more noise... both unwanted options for the intruder who usually wants to enter the premises, grab your possessions and get out as fast as possible.
Cement or concrete insulation barriers and an inner steel liners are an essential deterrent to defeating most any intruder from successfully breaking into a safe or vault. They add little additional cost and provide a great deal of additional security.