I guess the difference is in the contract, that when element can not be added to collection the add method throws an exception and offer doesn’t.
From: http://java.sun.com/j2se/1.5.0/docs/api/java/util/Collection.html#add%28E%29
If a collection refuses to add a
particular element for any reason
other than that it already contains
the element, it must throw an
exception (rather than returning
false). This preserves the invariant
that a collection always contains the
specified element after this call
returns.
From: http://java.sun.com/j2se/1.5.0/docs/api/java/util/Queue.html#offer%28E%29
Inserts the specified element into
this queue, if possible. When using
queues that may impose insertion
restrictions (for example capacity
bounds), method offer is generally
preferable to method
Collection.add(E), which can fail to
insert an element only by throwing an
exception.