Role of dam methylase in bacteria
The dam methylase has three different functions: 


Correction replication errors, since the new DNA molecule
is only hemimethylated (the old strand is methylated, the newly
synthesized is not). Since the proof-reading only takes place on the
new strand, errors introduced during replication can be corrected.
Regulation of replication: The ori of the chromosome is methylated to ensure that is only replicated once.
Regulation of transcription: Methylation of GATC sequences promote the transcription of genes.


For further details see the references.

References:


The great GATC: DNA methylation in E. coli
The dam and dcm strains of Escherichia coli--a review.
Dam methylation: coordinating cellular processes.