If, for example, you leave your company for a better position elsewhere, you will need to rollover the funds from your 401k account. (thecollegeinvestor.com)
You may be able to transfer the funds from your previous employer plan to a new employer plan (if it accepts rollover funds). (ufcu.pfyfn.com)
Technically I could roll it in with my 401k rollover funds but I like keeping it separate and seeing it grow. (chiefmomofficer.org)