Why do roots exist?

After attending my local fish club meeting this evening, I decided to post
a few questions that have been puzzling me for a while.

1. There are many plants that may or may not put roots out at their nodes.
Two examples that I have are a few Hyrophila species, and anacharis
(&/or elodea?).  What seeems to govern when these plants decide to put out
a thousand roots everywhere is beyond me.  Many strands of Elodea have
never sent out a root, and some have streamers al over them.  Some
hygrophila I have hasn't produced a single root, but in a tank I have with
soft, acidic water, peat/soil/sand sustrate, and fairly good light, the
tank is solid with hygrophila, and I'd say I have 60% leaves and 40%
roots.  Doesn't this seem odd?  According to our speaker thisevening,
aquatic plants absorb most nutrients from their foliage and have no need
for a major root system with root hairs, etc. the wat terrestrial plants
do.  Why then, do plants which could be "floaters and drifters" choose to
put out a thousand roots at every node?

2. Is this mysterious factor (that causes my hygrophila to decide to
produce pounds upon pounds of roots) related to the decision a plant
makes, on whether to bud/produce offspring rather than grow into one large
plant? For instance, under some circumstances, water sprite/wisteria will
produce a large "vegetative" body, and under other circumstances, baby
plants will appear at almost every nook in the plant leaves, and the
parent plant will disintigrate and die.  These events are so mysterious to
me that I'm tempted to create a religion to explain them!!

3. The success of liquid supplements would seem to suggest that the above
claim is true - that plants can absorb nutrients through their foliage,
and don't depend on roots for that - that perhaps roots are chiefly there
to help anchor the plant.  How does this claim rest with you all?  If this
is a reasonable statement, why is substrate that important - why bother
with laterite/soil etc. when you can just grow plants in gravel and
supplement the water with nutrients?  I'm just curious, but these
statements just don't seem to mesh with everything I've learned until now. 
Other members of my club swallowed these claims quietly though, which is
why I'm bringing ithem up here. Thanks for any ideas you might have on

- Jessica Dion, in soggy Vermont where my tomato plants are drowning and I
                could grow water sprite in my backyard garden!