the necessity of dilutng the concentrated acid/alkali.

when the concentration is high, it is not accurate to determine
the end point. for example, 2M of sodium hydroxide react
with 2M hydrochloric acid. when the titration is near endpoint,
it is not easy to reach the endpoint. it is easy to drop in excess
volume. but for , say 0.1M, we can control it better.
since the concentration is 20 times less than previous now.
the endpoint is easier to be found.
another point is that when the concentration is high , the chemical is
more corrosive, our body will be hurt when there is spillage

It is not the case that said by 'snakewongcm'
If you want to determine whether the reactants reach the end point you can shake the flask for a longer time to see the colour of the solution.It is not harmful that the chemical is corrosive. If there is spillage ,IT IS YOU FAULT THAT YOU DO NOT PERFORM THE EXPERIMENT CAREFULLY.Otherwise you must not do any experiment which involve coroisive chemicals!!
Dilute the known concentration of concentrated acid/alkali is to save chemicals,also,more titration can be done to obtain a more accurate result.
Also, a large amount of acid /alkali is needed to neutralize acid /alkali used in the experiment.Which also means that more acid/alkali is needed to change the colour of indicator used in the experiment.

2008-03-24 12:25:48 補充：
Otherwise you must not do any experiment which involve coroisive chemicals!!
改為
Otherwise you must not do any experiment which involve corrosive chemicals!!