Chain guides keep tension on the chain and keep them (or at least reduce the likely hood) from jumping gears.
They do not replace a front derailleur. If you plan on have two chain rings up front you will need a front derailleur and a two ring guide such as the E-13 DRS, MRP LRP, or Blackspires stinger. The two ring guides are more chain tensioners versus true chain guides.
If you run a single ring up front, you do not need a front derailleur and you have many options for a true chain guide.
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