It can hold my flower pots which are quite heavy.

The holes are for optional stoppers, so things don't accidentally slide and fall.

I used a bit of glue and 3x6mm drywall screws to wall mount it, the glue gives it extra strength so the screws don't unscrew in time, I use glue for most of wall mounted things that require strength.

The two ridges are for extra strength, and the two holes in the ridges are for a thin rod you can use to hang stuff on.

I needed mine to be really strong, printed 3 of these so far in PLA, works really well, and are very strong with these settings:

Hotend: 220C; layer height 0.2mm; no supports; cooling fan: 30%; perimeters: 4, solid top layer: 3, solid bottom layer:3-4 (this is where stuff sits on, so 4 is better for strength but 3 was fine for me; infill: 10%; infill pattern: cubic; 40mm/s speed on everything, I printed it very slow because slow, may give it extra strength, probably, I'm not really sure.

EDIT: I printed it in PLA, 200C, 130mm/s and it has the same strength, don't need to print it slow it seems.