The practice of asking an author to “transfer” a manuscript to another journal for consideration for publication has been used in the medical field for quite some time. It can occur (fairly) often when there exists a “cluster” of journals managed owned by the same group. An example is the American Medical Association, which publishes the Journal of the Medical Association (JAMA) and 11 other more specialized journals under the JAMA “umbrella” (e.g., JAMA Cardiology; JAMA Dermatology; JAMA Neurology). The JAMA editor assigned to a manuscript submitted to JAMA might ask the author if the manuscript (and reviews) can be transferred for consideration by, for example, JAMA Pediatrics if the JAMA editor believes that the manuscript might be of interest to the editor(s) of that journal. The referring editor makes no promise of acceptance by the journal to which the transferred paper is sent.
As indicated in a comment, there is an implicit presumption that the manuscript will be revised to address the comments of the (transferred) reviews. Sometimes the referring editor states this explicitly when asking about the transfer.
The “family of journals” with the prefix BMC, which is part of Springer Nature, publishes an:
“evolving portfolio of some 300 peer-reviewed journals, sharing
discoveries from research communities in science, technology,
engineering and medicine.”
The BMC family of journals has been quite aggressive in its attempt to modify the system of peer review. These are described at their website:
The things they are doing with regard to innovations in peer review include:
• Patient peer review • Registered Reports • Results-free
review • Automated peer review • Re-review opt out • Portable
peer review within and between publishers • Expedited peer review
The journal that asked about transferring your paper to another journal appears to be using “portable peer review.”
For the medical journal BMC Medicine, portable peer review is described as follows:
“Portable peer-review To reduce time spent on serial submissions and
iterative reviewing, BMC Medicine offers to consider manuscripts on
the basis of reviews received at other journals. We also support
transfers of reviews obtained at BMC Medicine to other journals,
including those outside of BMC and Springer Nature. Learn more from
our portable reviews page.”
The advantage to an author in permitting the transfer is a possibly shorter time to publication. The author also has a good idea of what they will need to do to get the manuscript accepted. It is quite common to revise a manuscript based on the comments of the reviewers chosen by journal 1 (this is a good practice) only to have a whole different set of comments that need to be addressed when the manuscript is submitted to a journal 2.
The advantage to a journal (or set of journals) is a reduction in the burden the journals put collectively on peer reviewers. The advantage to a journal (or family of journals) is that it helps assure a “stream” of potentially publishable manuscripts at the second (referred to) journal.
If the journal that is being suggested as a referral journal is not one that you would be proud to publish your work, say no.
If it is not possible (or desirable) to address the comments of the original reviewers, say no (because the paper is unlikely to be accepted unless these comments are addressed).
If the journal that is being suggested as a referral journal is a journal that will charge a lot of money to publish your paper, say no.
Finally, in medicine (and other fields), there has been a proliferation of “predatory journals.” The definition of a predatory journal is controversial, it is not easy to identify "predatory journals" reliably, and what is predatory to one person is not predatory to everyone. Nonetheless, it would be wise to do some digging to determine whether the referral journal might be predatory.
This publication from the medical field is an empiric study that attempted to define the criteria for a predatory journal formally. These criteria might not map easily to philosophy journals.