Ah. Well, I think you're understanding the card symbology, but not the function of the icons on the left side of each card. Those are Skill Icons and only come into play if you play the card to modify an existing skill test. So, if you were required to do a skill test, you could discard Dynamite Blast and get +1 to your Will test. The active player can discard as many cards as they like, whereas the other players may only discard 1 card when it's not their turn.
Page 8 of the LtP:
Modifying Skill Value for Skill Tests:
Before drawing a chaos token for a skill test, the investigator may boost his or her skill value. There are two ways to do this.
First, the investigator may commit eligible cards from his or her hand to the test. An eligible card bears one or more icons matching the skill type of the test being performed. A wild icon (?) matches all skill types. Each matching icon committed to a test increases the investigator's skill value by 1 for that test. The investigator performing the test may commit any number of cards from his or her hand to the test. Each other investigator at the same location as that investigator may commit 1 card from his or her hand to help.
Do not pay a card's resource cost when committing to a skill test...
Thus, that icon does nothing if you're playing Dynamite Blast as the event "Dynamite Blast", it only comes into effect if you want to discard it to boost a Will skill check. This is the most "gamey" part of the design, because it keeps all investigators involved even when not their turn and offers some difficult decisions. You really need to pass this Will check, but you had plans for that Dynamite later. Do you discard it, or take your chances without the +1 on the Will check? It becomes an even harder decision when it's not your turn and the other players do not know what cards you have in your hand. They may beg for some help on a Will check, and you'll have to tell them you can't because you REALLY want to use the Dynamite next turn. Fun stuff.