Since I write for magazines, of course I advocate people reading them!
But seriously, the best thing to do is buy a copy of one and see what you think! They're all different, but some, particularly PFK (in the UK) and TFH (in the US) are aimed at enthusiastic beginners and casual hobbyists. So you tend to find articles either about the most popular sorts of tanks and their species (community tanks and reef tanks especially) or else describing groups of fish that the majority of aquarists will encounter in their local tropical fish shops (cichlids, livebearers, etc.). One of the big plusses to magazines is the photography, and so even if you think you know about, say, barbs, a nicely illustrated article will show you species you haven't thought of keeping before, and maybe give you ideas of barbs you could try out some time. Some magazines have an "in the shops" type feature that tells you about new species in the trade, and this is a great way to learn about fish you've never heard of before.
Some other magazines are aimed at more advanced hobbyists. Coral Magazine is one such. Then you have the periodical magazine like the BowTie seasonal magazines about ponds and suchlike, which are more like mini-books than magazines.
It's true the internet has become a place for the exchange of information, and compared to the situation when I started keeping fish in the 80s, magazines are not as obviously essential. But as others have pointed out the quality of writing in magazines is much higher, and you can't really compare reading a piece by, say, Bob Fenner on marine fishkeeping or Ad Konings on cichlids with any random article that just comes up in Google. This isn't to say there isn't some excellent writing online; there are several websites that are not only free but top-notch in quality. But if you're a relative newcomer to the hobby, it's hard to tell the good stuff from the dross.
At least one magazine (TFH) has some articles from the magazine reproduced online in their entirety, so you may want to peruse some of those. PFK sometimes makes content from the magazine available online in the form of articles or blogs. Other magazines may do similarly. Have a read, find one you like, and enjoy!
Cheers, Neale
Does anyone on here advocate buying magazines? Are they useful - will I learn useful information as all?
