They can pray for them? Do hajj/Sadaqa on their behalf. They can help them physically?
When a person is dead he leaves this world. their grave is just there to remind us of him and of the reality of death, and remind us to pray for him. If we started to making a grave more than it is, then we will planting the seed of Shirk, it starts as a good/holy person who lived a good life, then people turn his grave into a shrine> Turns into a holy site> People visit and ask for blessings and favors (when they shouldv'e asked God) > After a long time and many generations, .people forget that it was just a grave. It becomes and Idol of some sort> they start idolizing the Shrine or whomever behind it.
All idols that were in Mecca, like Allat, ozza, were good religions people that lived long before the prophet (PBUH) was born.