Normal grouts are cement based (cementitious). Because of this, they absorb water easily. This unfortunately means that they absorb stain causing liquids easily as well. Sealers greatly reduce this by microscopically filling in the voids where the stain sets.

However...many tile jobs do not get sealer. Either the original tile installers do not seal the grout after installation, or homeowners don't keep up with preventative maintenance.

Even if this is the case, grout can be cleaned with success in many cases. Professional tile cleaning will vastly improve the look of the grout. If grout cleaning doesn't get the grout perfectly even, there are also epoxy grout colorants that can be applied.

Here is an example of a Saltillo floor that was not sealed or professionally cleaned for over 20 years until we came out there and spruced it up.