The term "suggestiveness" in relation to a definition of literature probably can be best paralleled by the phrase "open to interpretation."
In pulp fiction or genre fiction, there is very little left open to doubt - usually - and the text deals with plot and action centrally.
However, in "literature" we have many, many examples of texts which are rich in possible meanings and multiple interpretations regarding motivation, character and symbolism. The meaning of a text is suggested as opposed to being well-defined.
For instance, in The Stranger we are fully aware of what happens, but we don't necessarily know why it happens. The why is central to the text as we try to decipher Mersault's reason for committing murder. That why is elusive and the answer is only suggested in the text.
It is this element of interpretability that can lead to great discussions.
I think that "suggestiveness" in literature is important based upon the fact that the writer is not outright telling a reader how to react or feel. Instead, if a text is suggestive, the text will provide the reader with something to consider (and perhaps lead to a new or more refined way of thinking). If a text is blatant, a reader could be put off and not gain the knowledge or understand the point the author is trying to make.