Updated Jul 13
Some phages may be aggressively lytic (such as T4), and are possibly just parasitic.
However, phages that are slower-acting, and are present for an extended time period without killing the host, may have some kind of beneficial contribution.
The CTXφ bacteriophage infects Vibrio cholerae. The phage DNA is spliced into the host chromosome (as a “prophage” which isn’t necessarily making any new particles at the moment).
CTXφ contains the gene which codes for the cholera toxin, That toxin is what induces a human to have massive diarrhea during a cholera infection. That spews out lots of Vibrio cholerae, thereby helping the host to spread.
There is the Ff family (f1, fd, or M13), which keeps its genome separate, but secretes copies of itself without immediately lysing the host. I have read a couple of obscure mentions of some metabolic changes in the host. Although I don’t know if they are beneficial in any way.
I think another possibility is Lambda λ, which integrates as a prophage, which protects against further infection by any more Lambda λ copies. I suspect that maybe it also protects against other, more aggressive phage species.
Also, phages might be beneficial in the big picture. When prophages are being copied, the process isn’t perfect, and sometimes short segments of adjacent host DNA sequences are accidentally packed into new phage copies. This may result in horizontal gene transfer. That genetic information from the dead host is then injected into other bacteria.