tal:blocktal:block is a syntactic sugar for elements which contains many TAL attributes which are not to be echoed.
<tal:block define="myvar string:Some value"/>
is the same as:
<span tal:define="myvar string:Some value" tal:omit-tag=""/>
Another example:
<tal:block condition="someCondition" repeat="item someRepeat">
<div metal:use-macro="x"/>
</tal:block>
is the same as:
<div tal:omit-tag=""
tal:condition="someCondition"
tal:repeat="item someRepeat">
<div metal:use-macro="x"/>
</div>