This is not a flashy signing, but Dahl has the potential to help in multiple ways.
Dahl, 27, has started 40 games for the Rams over the past four seasons, including all 16 in 2012. He's better against run than pass and has played quite a bit on special teams. His addition gives the 49ers improved depth at safety.
I'm not sure if the 49ers view Dahl as a likely starter. He would appear to be part of a lower-cost approach to the position. If the 49ers had been willing to pay top dollar for a safety, they presumably would have re-signed Goldson or named him their franchise player for a second consecutive season.
Strength in the front seven should allow the 49ers to succeed on defense without overspending for their secondary. The team has already invested heavily at linebacker, with excellent results.
Dahl's departure from the Rams comes as no shock. The team released veteran starter Quintin Mikell before letting Dahl reach free agency. The safety market has been flooded with veterans. The Rams also have options in the draft. They hold two first-round picks and eight choices overall.