Because dealing with your mail can be so much better.

"Not much mail" is what Notmuch thinks about your email collection. Even if
you receive 12000 messages per month or have on the order of millions of
messages that you've been saving for decades. Regardless, Notmuch will be
able to quickly search all of it. It's just plain not much mail.

"Not much mail" is also what you should have in your inbox at any time.
Notmuch gives you what you need, (tags and fast search), so that you can
keep your inbox tamed and focus on what really matters in your life, (which
is surely not email).

Notmuch is an answer to Sup. Sup is a very good email program written by
William Morgan (and others) and is the direct inspiration for Notmuch.
Notmuch began as an effort to rewrite performance-critical pieces of Sup in
C rather than ruby. From there, it grew into a separate project. One
significant contribution Notmuch makes compared to Sup is the separation of
the indexer/searcher from the user interface. (Notmuch provides a library
interface so that its indexing/searching/tagging features can be integrated
into any email program.)

Notmuch is not much of an email program. It doesn't receive messages (no
POP or IMAP support). It doesn't send messages (no mail composer, no
network code at all). And for what it does do (email search) that work is
provided by an external library, Xapian. So if Notmuch provides no user
interface and Xapian does all the heavy lifting, then what's left here? Not
much.