The idea is to please Guru Sahib. If Guru Sahib gets pleased, then he does Naam di Kirpa. When Gursikhs are happy, they give Aseesa (blessings) and these blessings bring Guru Sahib's kirpa.
Normally, Gursikhs come home and do paath or kirtan and as a part of Gurmat Samagam, Guru ka Langar is served to the Sangat. This again results in getting khushi of the Sangat and Guru Sahib.
Now a days the idea of serving 5 Singhs has taken a different shape. Normally 5 Raagis from Gurdwara Sahib are invited to eat Parshada and most of the times ones who serve Parshada are not Gurmukhs following Gurmat Rehit. The Raagis who eat food this way, don't follow maryada either i.e. they don't discriminate what they eat. They don't care who has prepared the food or whether paath was done while preparing food or if Gurmat Maryada was followed. They just eat food and then do Ardaas for chardi kala of the family. There must be some reward in such sewa but it is nothing close to having a Gurmat Samagam at home and then serving the Sangat with Guru ka Langar (prepared with Maryada).
Kulbir Singh
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥