A hand full of texty cards is laborious and unsightly. Icons let a player see important information at a glance.
If you find yourself using the same text on numerous cards, you should turn that text into an icon.
However, there needs to be a relatively high cutoff for this. Yes, it's nice to make lots of icons, and simplify each card as much as possible, but your game should not descend into icon hieroglyphics, where there's an icon for everything.
In Wingspan, many cards give you eggs, food, or cards. However, the game uses almost no icons in the rules text of its cards. What could've been a clear and simple game now requires reading, because "draw a card" is written out on cards endlessly, instead of there just being an icon for it. Don't try to work out what other people's birds do — they'll have to read them to you, or hand them to you.
Icons are only about making things simpler for the player. They are not art, or a challenge for the designer.
An icon shouldn't be an miniature exact representation of what it does. It should depict one object, not be a medley of concepts. Just evoke the concept of the action. If "draw a blue card" is an action, you just show a blue card. You don't need to show an arrow removing the card from the deck. Players will remember what the icons mean.
In my gangster game, you can visit locations. Some
locations let you put a "Closed" sign on a location of your choice,
which means that no one can go there. This is a simple and interesting
ability. I made this ability into an icon. However, there are also a few
self-closing locations, that put the marker on themselves. I made a
separate icon for that ability.
That was a mistake. People constantly got confused as to which icon
was "Close any location", and which was "Close this location." Adding
arrows and other funny business simply made the icon complex, and people
still interpreted it incorrectly. I removed the "self-closed" icon, and
just turned it back into text, reading "put the
(closed marker icon) here." It was less elegant, but it was the correct
choice.
Keywords are like icons, except that they're a package of rules
condensed down into a word, instead of a picture. Use keywords only
where the word is going to be used as part of sentences. Otherwise, use
an icon. A picture is
much nicer than a word, which just blends into all the other words.
I see so many games using keywords instead of icons, because Magic does it. Magic came out in the 90's, as a low-budget game that took off. It can't easily switch to icons. You can.
Many times, a designer has told me that they want to be able to use their game rules in all kinds of different ways, and that standardising it into an icon would restrict them! Therefore, they write out the whole sentence "...this card deals three lightning damage..." on fifty cards in their game, rather than just having a lightning bolt icon with a "3" on it. "What if I wanted to make a card that does 3 lightning damage to all targets???" This is an abysmal trade-off, that makes a game complex and ugly, so that the designer can open up a small amount of unnecessary design space.
Magic makes thousands of cards, yet it has standardised all its most common abilities (albeit into keywords, not icons.) You can do it, too.
If you do want to escape your icons, you can still do so, by
modifying them with text, that overrides their standard rules.
In Radlands, the damage icon is very common. The
Sniper
has it, but she can hit anything. The card just has the damage icon,
followed by the text "any card". Radiation says "(damage icon) all
people." This keeps the cards in Radlands very clean and easy to parse.
You can also easily see which of your cards can do damage (or other
abilities.)
Another bonus is that when you turn text into an icon, it makes it inherently simpler. You should then use your icon in a lot more places, because it now has an excellent Depth:Complexity ratio.
A good game has most of its text reduced down to a small group of super-interesting icons.