If something is retentive, it is able to hold something else, so be glad if you have a retentive mind and grab a sponge if you need something retentive to sop up water.
Retentive is closely related to the word, retain. Both words come from the Latin retinere which means to hold back. If you are a farmer, it's good to know how water-retentive your soil is, or how much water it can hold and store. How much information can your brain hold and store? For someone with a retentive mind or memory or eye, it's quite a lot!