There are three ways to interpret this question.
You could be asking "what is the difference between the two directions of implication?".
You could be asking "what, intuitively, is the property of continuous functions captured by the real definition that is not captured by the backwards one?"
You could be asking "what are the consequences of the backwards definition?"
The answer to question 1 is that they are unrelated. To understand how, practically, they are unrelated, is answered by 2 and 3, but from the logical perspective, there is really no connection between $A \implies B$ and $B \implies A$.
The answer to 2 is what everyone always says about continuity: it is supposed to be the property that "values of $f$ at close values of $x$ are close". Presumably you have seen the informal "derivation" of the $\epsilon\delta$ definition from this prescription, but here it is again.
First, we have to express what it means that two numbers $x$ and $y$ are close; let's say that we have an "expectation" of closeness given by a number $e > 0$, and that their proximity "meets our expectation" if $|x - y| < e$.
Now we have to ask how to express the relationship between the closeness of two inputs $x$ and $a$ and of their corresponding outputs $f(x)$ and $f(a)$. We should establish an expectation, $\epsilon > 0$, for the proximity of $f(x)$ and $f(a)$, and depending on $f$ and $a$ we develop an expectation for the required proximity, $\delta > 0$ of $x$ and $a$ to make it happen.
Finally, we write out the chain of causality: we start with $\epsilon > 0$ and find $\delta > 0$, and if $x$ and $a$ meet the expectation established by $\delta$, then this (by intention) should result in $f(x)$ and $f(a)$ meeting the expectation established by $\epsilon$. Or in symbols:
$$\forall \epsilon > 0, \exists \delta > 0 \text{ s.t. } |x - a| < \delta \implies |f(x) - f(a)| < \epsilon.$$
There are actually two places where the chain of causality affects the order of things in the symbolic expression:
Starting with $\forall \epsilon$, or in words, starting with an expectation for the proximity of $f(x)$ and $f(a)$. The backwards version has this order also. However, to justify this, let me say that if you were to try to establish an expectation for the proximity of $x$ and $a$ first, you'd be trying to influence the proximity of the inputs of $x$ based on the proximity of its outputs, which is he opposite direction that functions work (input to output).
Making $x$ and $a$ satisfy their proximity expectation before $f(x)$ and $f(a)$. This is reversed in the backwards version, and the reason this is wrong is that if we say first that $f(x)$ is close enough to $f(a)$ without having found $x$, then we are making a blanket statement about any $x$ with the same value under $f$. It's like saying that you hate olives without having tasted the ones you are being offered.
Hopefully, this sheds some light on why the formal definition is taken to be exactly the way it is, in order to match the intuition.
Finally, we come to question 3: what does the backwards definition actually describe? The answer: every function. Because no matter what $\epsilon$ is, so long as we have $|f(x) - f(a)| < \epsilon$ just choose $\delta = 2|x - a|$, and then we have $|x - a| < \delta$. (And of course, if we don't have $|f(x) - f(x)| < \epsilon$, then we don't need to bother verifying the implication at all!) Speaking informally, this is because we not only get to choose $\delta$ after $\epsilon$, we also get to use it afterwards, so the statement with $\epsilon$ in it has no effect. In logic terms, we have both $F \implies T$ and $T \implies T$, so if you get to choose the right-hand side of an implication, you can always arrange it to be true. It's the left-hand side that's hard.